Active antenna system

ABSTRACT

The present invention relates to an active antenna system. The system has a plurality of antennas (2) and a radio equipment (3) connected to the antennas (2) and being located adjacent to the antennas (2). This radio equipment (3) comprises for each antenna (2) a separate transceiver module (12).

RELATED APPLICATIONS

This application is a Division of U.S. application Ser. No. 16/049,398,filed Jul. 30, 2018, which is a Division of U.S. application Ser. No.15/109,095, which is a § 371 National Phase Application of InternationalApplication No. PCT/EP2014/079483, filed on Dec. 30, 2014, which claimspriority to European Application No. 13 199 797.5, filed on Dec. 30,2013, all of which are incorporated herein by reference in theirentirety.

The present invention relates to an active antenna system.

Known active antenna systems comprise a plurality of antennas which areconnected to a radio equipment. The radio equipment comprises a singletransceiver for transmitting and receiving the antenna signals. Thetransceiver is connected to the individual antennas by means of apassive array. The radio equipment is connected by means of a data lineto a radio equipment control unit. The radio equipment control unitprocesses the data signals which are to be transmitted and which arereceived comprising decoding and encoding, distributing of the data tothe individual antennas, beamforming, etc.

US 2012/0196545 A1 discloses an antenna array having a plurality ofantenna elements. The antenna array comprises a plurality of transceivermodules, an active antenna element subset of the plurality of antennaelements, wherein the active antenna element subset comprises at leastone active antenna element being actively coupled to an associatedtransceiver module of the plurality of transceiver modules and at leastone passively combined sub-array of at least two antenna elements of theplurality of antenna elements. A method for generating antenna patternswith the antenna array is also disclosed.

US 2012/0243468 A1 describes a compression/decompression method forbackhaul communication of a complex-valued radio signal between basestations and network processing units, such as a Central Processor of aCoordinated MultiPoint (CoMP) system, significantly reducing backhaulbandwidth. The spatial and temporal correlations of a wireless IQ signalare exploited in order to remove redundancy and substantially reducesignal bandwidth. Feature component signals of significance areextracted through linear transformation to form the radio signal, andare individually quantized, possibly at different bit rates inaccordance with their relative importance. The transformation can eitherbe predetermined or computed in real-time based on the spatial andtemporal statistics of the radio signal. In the latter case, thetransformation matrix or matrices are also sent over the backhaul inorder to allow the radio signal to be reconstructed at the receivingend. Different methods of generating the transformation matrices areproposed.

US 2012/0190389 A1 discloses to compress multi-antenna complex-valuedsignals by exploiting both a spatial and a temporal correlation of thesignals to remove redundancy within the complex-valued signals andsubstantially reduce the capacity requirement of backhaul links. Afterreceiving the compressed signal at a receiver, a decompressordecompresses the received signal over space and over time to reconstructthe multiple antenna stream.

EP 2 632 058 A1 discloses an apparatus, a method and a computer programfor providing a composite beampattern, the apparatus providing acomposite beampattern for at least two antenna elements forming anantenna array coupled to at least two signal branches of a signal, thecomposite beam-pattern having at least two main lobes pointing indifferent spatial directions. Further, an adaptive antenna array with aninternal digital interface of base stations between Radio EquipmentControllers (REC) to local or remote radio units, also known as RadioEquipment (RE), is described. Between the REC and the RE multiplexeddigital base band signals are transferred over serial links using theCPRI-standard.

US 2012/0128040 A1 describes a module for an active antenna system forreceiving and transmitting radio signals sealed in a housing. In oneaspect of the invention, the module also includes a hub that controlsthe collection and distribution of the digital radio data and controldata. The hub may be switchable on and off so that in someimplementations of the invention a single central hub controls thecollection and distribution of the digital radio data and control datain different ones of the modules. In another implementation of theinvention, each one of the modules has its own hub for controlling thecollection and distribution of the digital radio data and control datain its own module as well as between the modules. Between the RadioEquipment Controller (REC) or base transceiver station, respectively,and the Radio Equipment (RE) or C-Hub in the antenna housing,respectively, multiplexed digital base band signals are transferred overserial links using the CPRI- and/or OBSAI-standard.

GB 2 440 192 A discloses a digital radio unit directly connected to aplurality of antenna elements forming the antenna. Each one of theantenna elements is thereby directly connected to the digital radio unitor its components. The digital radio unit converts RF signals receivedand transmitted via the antenna elements from/to a mobile station intosignals according to CPRI or OBSAI standard/interface. These signals arethen transferred via optical fibres up to 40 km long to a digital radioserver located at a base station. The digital radio unit comprises atleast one antenna element, at least one micro radio, and at least onehub, also referred to as “C-hub”. Between the Radio Equipment Controller(REC) or base transceiver station, respectively, and the Radio Equipment(RE) or C-Hub at the antenna, respectively, multiplexed digital baseband signals are transferred over serial links using the CPRI- and/orOBSAI-standard.

US 2010/0093282 A1 discloses a multi-transceiver architecture foradvanced Tx antenna monitoring and calibration in MIMO and smart antennacommunication systems. An exemplary wireless transceiver circuit isshown including two individually operated transceiver modules accordingto the invention configured for use in a wireless MIMO spatialmultiplexing system, whereas a second RF signal received via a secondantenna connected to a reception chain of the second transceiver moduleis used for providing feedback information about a first RF signaltransmitted from a first antenna connected to a transmission chain ofthe first transceiver module via a closed antenna loop. Further, anexemplary principle of a calibration set-up for a smart antenna systemin downlink calibration during system manufacture (factory calibration)according to an exemplary embodiment of the present invention and anexemplary principle of a downlink calibration of the smart antennasystem which can be carried out when the exemplary system isactivated/reset (field calibration) or during the run time of the system(run-time calibration) are disclosed.

U.S. Pat. No. 6,801,788 B1 discloses a telecommunication base stationtransceiver subsystem that can be configured to provide signal ormulticarrier frequency services. The base station is divided into aradio unit, which is positioned proximate to the antennas, and a mainunit, which is remotely located from the radio unit. The transmissionbetween the main unit and the radio unit comprises an analog signalperiod.

U.S. Pat. No. 5,903,834 describes a communication network and associatedmethods, permitting wireless communication with a mobile unit in anindoor environment. Transceivers are positioned at spaced-apartlocations and are coupled to a centralized control device. Uplinksignals transmitted via a mobile unit are received by receiver portionsof transceivers within the range of the uplink signal. Downlink signalsare generated by at least two transmitter portions of at least twotransceivers.

US 2010/0278530 A1 discloses a distributed antenna system forcommunicating with a plurality of space stations. The distributedantenna system includes a system controller and a master unitcommunicating with at least one of the plurality of space stations. Aremote unit communicates over high data rate media with the master unitand/or a downstream remote unit. Alternatively, the distributed antennasystem includes a controller and a digital time/space crosspoint switchcontrolled by the controller. By digitizing the transceiver is in incommunication with the digital time/space crosspoint switch. Thecrosspoint switch is configured to transmit and receive digital datathrough the digitizing transceiver. For each RF band the master unitcombines the downlink signal from up to four base stations on a per bandbasis and digitalizes the combined signal. The digitized and combinedsignals from each of the RF bands may then be time division multiplexedinto frames and converted to a single serial stream.

US 2011/0150050 describes a digital integrated antenna array forenhancing coverage and capacity of a wireless network. The digitalantenna array comprises antennas connected to duplex filters connectedto amplifiers. Direction couplers between each antenna and therespective duplex filters as well as direction couplers between eachduplex filter and the respective amplifiers are used to calibrate thehardware as required for digital beamforming. All calibration signalsfrom the different direction couplers are fed by means of wireconnections into a beamforming core, which controls and calibrates aplurality of antennas.

WO 92/13400 A1 discloses a multichannel radiotelephonie systemcomprising an antenna connected to a duplex filter connected toamplifiers. A direction coupler between the antenna and the duplexfilter as well as a direction coupler between the duplex filter and theamplifiers are providing remote diagnostics and measurements as well asRF output subsystem fault detection.

US 2004/0219950 A1 discloses an antenna arrangement and base transceiverstation. The base transceiver station comprises at least one activeantenna connected to a local unit for performing conversion between alow-frequency digital signal and a radio frequency electromagneticfield.

The object of the present invention is to provide an active antennasystem which is very flexible configurable and efficient in operation.

The object is solved by an active antenna system according to claim 1.Advantageous embodiments of the present invention are disclosed in thecorresponding subclaims.

According to a first aspect of the present invention an active antennasystem comprises

-   -   a radio equipment having a plurality of transceiver modules,        each transceiver module being connected to at least one antenna,    -   a radio equipment control unit having a hub being connected to        the transceiver modules via an antenna interconnect,        wherein the hub is embodied for receiving base band signals via        the antenna interconnect from the transceiver modules and to        extract channel signals from the received base band signals.

As the hub is arranged in the radio equipment control unit which is partof a base station the base band signals are transmitted to the radioequipment control unit without extracting the channel signals. Thisallows easily to distribute the base band signals on several dataconnections, particularly several antenna cables, of one antennainterconnect. This results in a reduced information loss and minimizesthe requirements for the transmission capacity of a single dataconnection of the antenna interconnect between the radio equipmentcontrol unit and the radio equipment. The radio equipment control unitdoes receive all the information which is originally received by thetransceiver modules.

The hub separates channel signals of several channels transmitted viaone antenna. Therefore, the channel signals of one antenna are combinedduring the transmission via the antenna interconnect. A hub whichseparates the channel signals can also be integrated into a channelcard.

A further advantage of the hub in the radio equipment control unit isthat less power is needed in the radio equipment. As the radio equipmentis usually located at the top of an antenna tower the available power islimited.

According to a further aspect of the present invention an active antennasystem comprises

-   -   a radio equipment comprising a plurality of transceiver modules,        each transceiver module being connected to at least one antenna,    -   a radio equipment control unit,        wherein each transceiver module comprises a switch matrix being        connected to the radio equipment control unit via an antenna        interconnect, wherein at least two switch matrices are connected        by means of an intermediate cable.

Such an active antenna system allows to direct flexibly the data via oneor the other switch matrix. Preferably the antenna interconnectcomprises several antenna cables so that also the data stream isflexible distributed via the several cables by means of the switchmatrices.

A switch matrix is a data switch which decides on the content of datapackages, particularly the headers, to which target the package is to betransmitted.

The active antenna system according to a further aspect of the presentinvention comprises a plurality of antennas, a radio equipment connectedto the antennas and being located adjacent to the antennas, wherein theradio equipment comprises a separate transceiver module for eachantenna.

Thus, the present invention provides a distributed transceiver systemfor the plurality of antennas, so that the antenna signals of eachantenna are individually processed.

By providing a separate transceiver for each antenna, the signals foreach antenna can be individually processed. This increases theflexibility because the signals can be individually adapted for eachantenna. The distributed transceiver structure is a completely newarchitecture of the radio equipment of active antenna systems, whichprovides new solutions for a plurality of different aspects. Preferably,the radio equipment comprises an antenna module for each antenna,wherein each antenna module comprising a transceiver module is embodiedwith an analog-to-digital converter and a digital-to-analog converter.This allows a digital pre- and post-processing of the individual antennasignals in the radio equipment. Furthermore, this makes it possible totransmit and receive digital data to and from a radio equipment controlunit. These data can be compressed. Due to the distributed transceiverarchitecture, the signals of neighboring antennas are usually verysimilar, so that the compression is very efficient. It is particularlypreferred to jointly compress the digital signals of or for severalantennas.

The jointly compressed signals are preferably beamforming signalsbecause beamforming signals for several antennas differ mostly only inthe phase shift, wherein the data content is usually substantiallyidentical. Such kind of data can be compressed very efficiently, e.g. bytransmitting the content only once and transmitting the phase shift forthe individual antennas separately.

The radio equipment and the radio equipment control unit are preferablyconnected by an antenna connect. The antenna connect can be embodied asoptical fiber. The optical fiber can be a single mode fiber or amulti-mode fiber, so that data signals can be transmitted in multiplecolors according to the DWDM-standard.

In one preferred embodiment, a hub is provided in the radio equipmentcontrol unit, but no hubs are provided in the radio equipment. Such adesign is very flexible because the transceiver modules can beincorporated in any kind of topologies. This architecture is freelyscaleable and provides a high redundancy. The radio equipment comprisingno hub is simpler than ordinary radio equipment and comprises lesscomponents and circuits, thus it is more stable.

According to further embodiments, a control RF base band unit, which ispart of the radio equipment, comprises a switch matrix, wherein theswitch matrix is connected to an antenna interconnect. Additionally, theswitch matrix is connected to at least two terminals, wherein theterminals can be connected by means of an intermediate cable to acorresponding terminal of a switch matrix of another control RF baseband unit. Thus, several transceiver modules of the radio equipment canbe connected by these intermediate cables, wherein at least one or moreswitch matrix is connected by an antenna interconnect with the radioequipment control unit. With such a design, it is possible to provideparticularly suitable topologies, such as comprising only a few antennainterconnects for transmitting redundant data only once between theradio equipment control unit and the radio equipment and duplicating anddistributing the redundant data in between the several transceivermodules or antenna modules, respectively, wherein only individual phaseinformation is provided for each antenna module.

The data compression for compressing the digital data which are to betransmitted between the radio equipment and the radio equipment controlunit can comprise a lossless data compression or a lossy data reductionprocess.

Such kind of data compression is particularly efficient in combinationwith an active antenna system having a distributed architecture of theradio equipment, which means that an individual transceiver module isassigned to each antenna. Thereby, the digital signals of the differentantennas are very similar, so that the compression is very efficient.

In a beam forming mode it is possible to calculate once a phase shiftper time unit for a moving user equipment (UE) of a communicationpartner which communicates with the active antenna system so that thecorresponding phase shifts can be calculated continuously in the radioequipment and a beam can automatically follow the user equipment.

By means of the received signals the position of the user equipment canbe tracked and in case the user equipment deviates from the predictedtrajectory a corresponding correction of the phase shift can be carriedout.

According to a further embodiment the transceiver module comprises meansfor calibrating each transceiver module separately with respect to phaseand/or amplitude. Such a calibrating means can be provided for thetransmission path and/or for the receiving path.

The means for calibrating are comprising a feedback loop which providesa feedback of a reference signal which is picked up for the transmissionpath or which is coupled to the receiving path by means of a couplingelement adjacent to the antenna. A second, alternative coupling elementcan be provided at the inner side of a duplex filter.

According to a further example an active antenna system is providedcomprising

-   -   a plurality of antenna modules, wherein each antenna module        comprises an antenna, a transceiver module and a control        RF-based unit, wherein means for synchronizing the several        antenna modules are provided. These calibration means are        embodied to measure time delays between respective antenna        modules and to apply to each antenna module a certain delay so        that the runtime in between the antenna modules is compensated        for outputting the antenna signals.

The calibration means can be embodied for measuring the maximum delay inthe topologic. Alternatively, the calibration means can be embodied formeasuring only the delays to the neighboring antenna modules.

The invention is explained in the following in more detail by means ofthe enclosed drawings which show several examples of the invention. Thedrawings show in

FIG. 1 schematically an active antenna system,

FIG. 2 an antenna module in a side view,

FIG. 3 schematically an antenna array in a top view,

FIG. 4 radio equipment with a transceiver module in a block diagram,

FIG. 5 an antenna comprising several radiators in a side view,

FIG. 6a an active antenna system comprising several transceiver modulesconnected to a radio equipment control unit in a block diagram,

FIG. 6b a further embodiment of transceiver modules in a block diagram,

FIG. 7 a diagram of the electric field with two main lobs and four sidelobs,

FIG. 8a, 8b embodiments of Tx-transmission chips in block diagrams,

FIG. 9a, 9b embodiments of Rx-receiving chips in block diagrams,

FIG. 10a, 10b synchronizing means for a daisy chain schematically in ablock diagram,

FIG. 11a, 11b schematically synchronizing means for a tree-topology,

FIG. 12a, 12b antenna modules having means for synchronizing clocksignals,

FIG. 13a, 13b an amplification unit for simultaneously amplifying andconverting a digital input signal to an analog output signal;

FIG. 14 a transceiver module comprising a IQ-compensation module, and

FIG. 15-23 different embodiments of the radio equipment based on opticaldomain.

An active antenna system (AAS) 1 comprises a plurality of antennas 2.The antennas 2 are part of a radio equipment 3. The radio equipment 3comprises control circuits for controlling the signals which are to betransmitted via the antennas 2 and for controlling the signals which arereceived by means of the antennas 2. Examples of the radio equipment 3will be explained below in more detail.

Each antenna 2 is connected to a transceiver module 12 and can consistof one or more radiators 99 (FIGS. 4, 5). Radiators 99 of one antenna 2receive substantially the same transmission antenna signal, becausethere are no active elements between the transceiver and thecorresponding radiators 99. It is possible that the radiators 99 receivetransmission antenna signals with a slight phase shift due to differentlength of the connections between the transceiver and the radiators 99.

The radiator(s) 99 of an antenna 2 can be cross polarized. In such acase two independent signals can be transmitted and received via oneantenna 2. Such a cross polarized antenna 2 is preferably connected totwo transceiver modules 12.

The radio equipment 3 is usually mounted on an antenna tower 4 (FIG. 1).

In the first embodiments of the active antenna system 1 a radioequipment control unit 5 is located distant from the radio equipment 3,wherein the radio equipment 3 and the radio equipment control unit 5 areconnected by means of one or more antenna interconnects 6. The radioequipment 3, the radio equipment control unit 5 and the antennainterconnect 6 are typically part of a base station 101 such as a Node Bin UMTS (UMTS: Universal Mobile Telecommunications System) or a basetransceiver station in GSM (GSM: Global System for MobileCommunications).

Typically the data which are to be transmitted with the antennainterconnect 6 are digital data. Basically it is also possible totransmit analogous modulated data. The data is typically transmitted bymeans of the CPRI standard (Common Public Radio Interface), which is aninterface standard comprising a transmission with the data beinginserted into frames as containers. Transmitting the data is alsopossible by means of any other appropriate standard such as for exampleOBSAI (Open Base Station Architecture Initiative).

The antenna interconnect 6 comprises preferably a plurality of opticalcable, particularly a single-mode or one or more multi-mode glass-fibercables, as antenna cables 10 for transmitting a plurality of datasignals simultaneously. Alternatively each of the antenna cables 10 canbe provided as an electrical cable, preferably as a coaxial cable.Preferably, each antenna cable 10 transmits the signals received and/ortransmitted by one single transceiver module 12. This provides thepossibility to control the signal each transceiver module 12 receivesfrom the radio equipment control unit 5 and to process all the receivedsignals from each transceiver module 12 in the radio equipment controlunit 5 together, which is described below in more detail.

On the contrary, in ordinary active antenna systems the received antennasignals are preprocessed and then multiplexed in a C-hub in the radioequipment, as disclosed in EP 2 632 058 A1, US 2012/0128040 A1 and GB 2440 192 A. Also, the transmitted signals are multiplexed in the radioequipment control unit before being transmitted to the radio equipment.This results in information loss and stresses the transmission capacityof the antenna interconnect between the radio equipment control unit andthe radio equipment. Due to the information loss, the radio equipmentcontrol unit does not receive all the information which is originallyreceived by the transceiver modules. The limited transmission capacityof the antenna interconnect prohibits high speed transmissions with lowlatency over the ordinary active antenna system.

It is also possible to embody the antenna interconnect 6 as a radioconnect using active antenna systems. The receiver and transmitter canbe tracked to each other so that swaying of the tower 4 can becompensated.

The radio equipment control unit 5 comprises an interface 7 which isconnected to a radio network controller (UMTS) or base stationcontroller (GSM) 69 for receiving and transmitting data (FIGS. 1, 6 a).The data comprises voice data, email data, sms/mms data, video data,music data, control data etc.

The radio network controller/base station controller 69 can be connectedto one or more wide area networks 100, such as the internet or a corenetwork of the UMTS. Further, the radio network controller 69 accordingto the UMTS standard can be connected to other radio network controllers69.

The radio equipment control unit 5 comprises a core controller 8. Thecore controller is the main processor which controls the modules of theradio equipment control unit 5. Furthermore, the core controller 8controls all processes in the radio equipment control unit 5, e.g.distributing the antenna signals to the respective antennas 2.

The radio equipment control unit 5 comprises a plurality of channelcards 9. The channel cards 9 are connected to the interface 7 and areembodied for receiving incoming data from the interface 7 and convertingthe data into channel signals on a base band frequency and vice versa.The channel cards 9 are connected to the antenna interconnect 6 directlyor via a hub 11 and thus to the transceiver modules 12. The channelcards 9 are embodied for transmitting the channel signals on the baseband frequency to the transceiver modules 12 and vice versa.

A logical channel signal comprises data of one or more logical channels,wherein a logical channel comprises data of one or more subscribers orcommunication partners. The channel cards 9 can further be embodied forprocessing the data, channel signals and/or logical channels, i.e.scrambling, descrambling, extracting the logical channels from and/ormultiplexing the logical channels to the channel signals, coding and/orfrequency conversion.

Each channel card can serve for 1 to 100 antennas 2. Usually the radioequipment control unit 5 comprises one to twelve channel cards 9 forhandling up to 1000 or even more telephone calls simultaneously. Thechannel cards 9 comprise a plurality of DSPs (digital signalprocessors).

As described above, preferably for each antenna 2 an individualtransceiver module 12 is provided. Each transceiver module 12 comprisesa combined transceiver and filter unit 13 and a control RF base bandunit 14 (FIG. 4). The transceiver filter unit 13 comprises a duplexfilter 15. It is also possible to provide two filter elements one fortransmitting and one for receiving antenna signals instead of one duplexfilter 15.

The duplex filter 15 is connected to a Tx-transmission chip 16 and aRx-receiving chip 17. The Tx-transmission chip 16 and the Rx-receivingchip 17 are connected to a RF-base band chip 18.

Basically, the RF-base band chip 18 converts data signals coming fromthe radio equipment control unit 5 onto a base band frequency. In theRF-base band chip 18 the base band signals are processed, wherein acrest-factor reduction and/or a pre- or postdistortion and/or afrequency conversation and/or another processing step for preparing thebase band signals to be converted to a high frequency signal is carriedout.

This base band frequency signal is converted to a high frequency signalby means of the Tx-transmission chip 16 which is a heterodyne orhomodyne architecture comprising converters, filters, and amplifier. TheRx-receiving chip 17 converts the high frequency antenna signal receivedfrom the duplex filter 15 to a base band frequency signal. Thus, theRx-receiving chip 17 and the Tx-transmission chip 16 are shifting theantenna signals to the base band frequency signal and vice versa.

The Tx-transmission chip 16, the Rx-receiving chip 17 and the duplexfilter 15 are elements of the transceiver filter unit 13. Thetransceiver filter unit is based on high frequency technology which isusually made by means of a CMOS, SiGe or GaAs-chip, wherein typicallythe Tx-transmission chip 16 is made of GaAs or GaN, the Rx-receivingchip 17 is made of SiGe oder CMOS and the control RF-base band unit 14is made of CMOS.

The control RF-base band unit 14 comprises besides the RF-base band chip18 several digital filter structures 19, a calibration module 20, acontroller 21, and a transport interface 22.

With the digital filters 19 the data signals can be preprocessed forbeing e.g. decimated, filtered or for converting the sample rate. Thecalibration module 20 is provided for calibrating the antennas 2 as itis explained below. In a transport interface 22 the incoming or outgoingdata are packed or unpacked in and from CPRI containers. Each CPRIcontainer can contain data for or from one, several or all antennas 2and for one or more logical channels.

The RF-base band chip 18 and the further elements 19-22 can be embodiedon a single chip which is e.g. based on silicon technology.

In the first embodiment (FIG. 6a ) preferably each of the transceivermodules 12 is connected by means of an antenna cable 10 with the hub 11being located in the radio equipment control unit 5. The hub 11 isfurther connected to the channel cards 9 and embodied to transfer thechannel signals from the channel cards 9 via the antenna interconnect 6to the transceiver modules 12 and vice versa by means of the CPRIstandard. Further, the hub 11 can be embodied to multiplex and extractthe channel signals and/or scramble and descramble the channel signals,which means that the hub 11 comprises partly the functionality thechannel cards 9 have as described above.

Each antenna cable 10 is preferably a glass-fiber cable, wherein thedata signals are transmitted in one colour or in multiple colorsaccording to the DWDM-standard (Dense Wavelength Division Multiplexing).This provides a high bandwidth for each antenna, so that a huge amountof information can be transmitted between the radio equipment controlunit 5 and the transceiver module 12 of the corresponding antenna 2. Incase of the antenna cables 10 being glass-fiber cables the hub 11 isembodied to convert electric signals into optical signals and viceversa.

If the antenna cable 10 can is embodied as a multimode glass-fibercable, then beam splitters (not shown) are provided at the input side ofthe transceiver modules 12 for directing the predetermined colour to therespective transceiver module 12.

The radio equipment 3 is free of any hub.

Such a design is very flexible, because the transceiver module 12 can beincorporated in any kind of topologies. The design is freely scalableand provides a high redundancy. The radio equipment 3 comprising no hubis simpler than ordinary radio equipments and comprises less componentsand circuits, so it is more stable. The radio equipment 3 is usually anoutdoor antenna which is exposed to all environmental impacts. The moresimple design provides a significantly better reliability. And sinceeach antenna signal of each antenna 2 and each transceiver module 12 istransmitted via the respective antenna cable 10, controlling the antennasignal in transmission direction in the radio equipment control unit 5is provided as well as further intelligent processing the receivedantenna signals as a whole in the radio equipment control unit 5.

Each antenna 2, transceiver module 12 and control RF-base band unit 14form an antenna module 23 (FIG. 2).

A plurality of antenna modules 23 form an antenna array 24 (FIG. 3). Inan antenna array 24 the distance between neighbouring antennas 2 orradiators 99, respectively, is typically about 0.5λ to 2λ. It is alsopossible that the distance between neighbouring antennas 2 or radiators99, respectively, in the edge region of the antenna array is larger than2λ for increasing the aperture of the antenna array and for suppressingside lobes.

As explained below the plurality of antenna modules 23 can be controlledin a beamforming mode so that the emitted beams are forming severalnarrow lobes.

Beamforming signals for several antennas 2 or radiators 99,respectively, differ mostly only in the phase shift, wherein the datacontent is usually substantially identical. Such kind of content datacan be transmitted once from the radio equipment control unit 5 to theradio equipment 3, wherein the phase shifts and/or amplitude shifts forthe individual antennas 2 are separately transmitted. Beamforming can bealso carried out for received antenna signals, wherein it is appropriatethat in the radio equipment control unit 5 a beamforming vector iscalculated and this beamforming vector is transmitted to the radioequipment 3 for carrying out the beamforming. For calculating thebeamforming vector it is appropriate to have the encoded and/orrescrambled content information which is usually only in the radioequipment control unit 5 available.

FIG. 6b shows a radio equipment of a second embodiment of the presentinvention. This radio equipment 3 is part of an active antenna system 1as described above. The same parts are designated with the samereference signs and are not again explained in detail.

This radio equipment 3 comprises several antennas 2 and transceivermodules 12. Each transceiver module 12 comprises a transceiver filterunit 13 and a control RF-base band unit 14. The transceiver filter unit13 is identical to the one of the above described embodiment. Thecontrol RF-base band unit 14 is similar to the one of the abovedescribed embodiment and comprises also an RF-base band chip 18, adigital filter 19, a calibration module 20, a controller 21, and atransport interface 22. Additionally the control RF-base band unit 14comprises a switch matrix 25. The switch matrix 25 can be connected toan antenna cable 10 of the antenna interconnect 6. Additionally, theswitch matrix 25 is connected to two or more terminals 26. The terminals26 of two different transceiver modules 12 can be connected byintermediate cables 27, wherein the intermediate cables 27 extendbetween two control RF-base band units 14. Thus, two or more transceivermodules 12 are connected by these intermediate cables 27, wherein (atleast) one (or more) transceiver modules 12 are connected by means of anantenna cable 10 with the radio equipment control unit 5. This one ormore antenna cables 10 are preferably optical glass-fiber cables,wherein the intermediate cables 27 are preferably electric cables. Theseintermediate cables 27 and the at least one antenna interconnect 6 forma data bus passing all control RF-base band units 14.

The switch matrix 25 is further connected to the elements 18-22.

The switch matrix 25 can be used in different arrangements. For example,all antenna modules 23 can be connected by an individual antenna cable10 with the radio equipment control unit 5 and additionally the data buscan be provided connecting the radio equipment control unit 5 and allantenna modules 23. By means of the data bus a broadcast message to allantenna modules 23 can be easily sent. This is particularly advantageousfor a booting and configuration process.

Furthermore, if a specific antenna cable 10 is blocked by a strong datatraffic or by a defect such as a broken connection or failing electronicdevices, then further information can be transmitted via a differentantenna cable 10 to another antenna module 23 from which the informationis transmitted via the data bus to the original antenna module 23. Thus,it is possible to re-direct traffic in case of a high data load or anyother blockade of an antenna cable 10 of the antenna interconnect 6.

Furthermore, flexibility of the antenna architecture is achieved inswitching antenna modules 23 together to serve different requirements.For example, it is possible to reduce power consumption of the radioequipment 3 by using only a subset of the antenna modules 23 fortransmission to a communication partner 63 (FIG. 1). Another example isusing different subsets of the antenna modules 23 to communicate withdifferent communication partners 63, whereby each of the subsets isexclusively provided for one of the communication partners 63. Againthis reduces power consumption but may also serve other purposes likesecure transmissions. These examples are provided that the consumedpower is sufficient for receiving and transmitting to the communicationpartner(s) 63.

Such a communication partner 63 is typically a mobile device, such as amobile phone, smart phone, computer, tablet, etc. comprising acommunication unit. Such a communication unit has one, two or even moreantennas, a transceiver for receiving and transmitting radio signals viathe antenna(s) and a digital processor unit for processing the antennasignals.

In this case of using switch matrices 25 the overall data trafficbetween the radio equipment 3 and the radio equipment control unit 5 canbe handled by a lower number of antenna cables 10 as installed antennamodules 23 so that only a limited number of antenna modules 23 areconnected by individual antenna cables 10 with the radio equipmentcontrol unit 5 and the other antenna modules 23 are connected only viathe data bus to other antenna modules 23 and to the radio equipmentcontrol unit 5. Such a switch matrix allows an individual configurationof the antenna modules 23 which can be adapted to the actually usedmethod of transmitting and receiving data signals via the radioequipment 3.

In this embodiment each antenna signal of each antenna 2 and eachtransceiver module 12 is transmitted via the respective antenna cable 10or another antenna cable 10 connected to a neighboring switch matrix 25.Thus, similar to the embodiment described above with the hub 11incorporated into the radio equipment control unit 5, controlling theantenna signal of each transceiver module 12 in transmission directionin the radio equipment control unit 5 is provided as well as furtherintelligent processing the received antenna signals as a whole in theradio equipment control unit 5.

Basically, in this embodiment the hub 11 in the radio equipment controlunit 5 can be integrated into the channel cards 9. Thus, the transceivermodules 12 which are connected to an antenna cable 10, are directlyconnected to the channel cards 9. In case of antenna cables 10 beingembodied as optical fibres, the conversion from electrical to opticalsignals and vice versa can be performed by simple converters (notshown).

The above-described embodiments can also be provided in combination inan active antenna system. In such a combination the above-describedadvantages of the invention are achieved, namely the high speed transferwith low latency over the antenna interconnect 6, providing all thereceived antenna signals to the radio equipment control unit 5, fullcontrol of the transmitted antenna signals by the radio equipmentcontrol unit 5, flexible configuration of the antenna architecture withlow power consumption by switching different antenna modules 23 togetheras needed, and fault tolerance by redundant provided antenna cables 10.

According to a further embodiment of the active antenna system the radioequipment 3 and the radio equipment control unit 5 are forming a singleunit (not shown). In this unit the radio equipment 3 is directlyconnected to the antenna modules 23. The radio equipment does not needthe transport interface. The antenna modules 23 are identical or similarto the embodiments shown in FIGS. 6a and 6b . A direct connectionbetween the radio equipment 3 and the antenna modules 23 means thatthese elements are connected by electrical conductor paths. Due to theclose arrangement of the radio equipment 3 and the antenna modules 23 itis possible to provide substantially any number of conductor pathsbetween the radio equipment 3 and the antenna modules 23. Theseconductor paths can be embodied as printed circuits on a common printedcircuit board of radio equipment 3 and the antenna modules 23. Theseconnector paths can be also short electrical cables. It is also possibleto embody the radio equipment 3 and the control RF base band unit 14 ona single chip. There is no need to transform the electrical signals intooptical signals and to transmit them via a low number of optical cables.

In ordinary active antenna systems the data rate between the radioequipment 3 and the radio equipment control unit 5 is limited to about10 GB/s. The smaller the distance between the radio equipment 3 and theradio equipment control unit 5 is, the easier it is to increase the datarate. Ordinary active antenna systems need an average power of about 10W for each antenna module just for transmitting the data signals betweenthe radio equipment 3 and the radio equipment control unit 5. Thus, thetransmission of the data between the radio equipment 3 and the radioequipment control unit 5 form a significant power load. The closer thedistance between the radio equipment 3 and the radio equipment controlunit 5 is, the less power is needed.

It is obvious to a person skilled in the art that with the presentinvention power consumption is decreased and transmission rates betweenthe radio equipment 3 and the radio equipment control unit 5 areincreased compared to ordinary active antenna systems, as describedabove.

The above described active antenna systems can be basically used in aMIMO mode (Multiple-in-Multiple-Out) or in a beam forming mode. In theMIMO mode each antenna 2 (or group of antennas 2) is sending an antennasignal which is physically independent of the antenna signals of theother antennas 2 of the antenna array 24. Physically independent meansthat the antenna signals are not physically linked but they can belinked by the data content. E.g. if several antennas 2 are sendingsimultaneously the identical antenna signal to achieve a high intensitythen these antenna signals are not physically linked. A certain amountof information can also be distributed in different sections, whereinthe different sections of information are sent by means of differentantenna signals. The information contained in the antenna signals islinked to each other but the individual antenna signals are physicallyindependent of each other.

In the beam forming mode the antenna signals of several antennas 2 arephysically linked with each other so that the beam has a certain form.This beam forming is achieved by emitting substantially identicalantenna signals with different antennas 2, wherein the antenna signalshave a certain phase delay to each other and/or a certain amplituderelationship, so that by constructive and destructive interference acertain beam is formed.

In the beam forming mode these antenna signals are physically linked toeach other by the phase difference and/or the amplitude relationship.

The active antenna systems can also be used in a combined MIMO mode andbeam forming mode in that several groups of antennas 2 are used foremitting certain beams and/or individual antennas 2 are usedsimultaneously for emitting an independent antenna signal. It is alsopossible to combine MIMO with beamforming, wherein certain MIMO signalsare emitted via several antennas 2 and the signals of the severalantennas 2 form a beamforming signal.

A very simplified static simulation of n emitting antennas 2 provides atwo-dimensional normalized relative intensity of the electric field of:

$\begin{matrix}{{E_{rel} = {\frac{\sin\lbrack {n( {\frac{\pi*d*\sin\mspace{14mu}\phi}{\lambda} - \frac{\alpha}{2}} )} \rbrack}{n*{\sin\lbrack {\frac{\pi*d*\sin\mspace{14mu}\phi}{\lambda} - \frac{\alpha}{2}} \rbrack}}}},} & (1)\end{matrix}$

wherein n is a number of the antennas 2, d is the distance of theantennas 2 in λ, ϕ is the angle with respect to the main emitting axisand α is the phase difference between neighbouring antennas 2 assuming alinear array with equidistant antennas 2. FIG. 7 shows a diagram of theelectric field, with two main lobes 28 and four side lobes 29. Forcalculating the intensity of the electrical field in the far field(>10λ) in the direction of emission ϕ for two isotropic antennas 2, thefollowing formulas can be used:

$\begin{matrix}{\Psi = {{d_{r}*\cos\mspace{14mu}\phi} + \delta}} & (2) \\{E = {E_{0}( {e^{j*\frac{\Psi}{2}} + e^{{- j}*\frac{\Psi}{2}}} )}} & (3) \\{E = {E_{0}\frac{e^{j*n*\frac{\Psi}{2}}}{e^{j*\frac{\Psi}{2}}}( \frac{e^{j*n*\frac{\Psi}{2}} + e^{{- j}*n*\frac{\Psi}{2}}}{e^{j*\frac{\Psi}{2}} - e^{{- j}*\frac{\Psi}{2}}} )}} & (4)\end{matrix}$

wherein ψ is the phase difference, d_(r) is the distance of adjacentradiators 99 in radians (d_(r)=2πd/λ), ψ is the direction of emissionand δ is the linear phase shift of adjacent radiators 99. These formulasform the basis for calculating the phase differences and the amplitudesfor a beam forming. However, mostly empirical weighting components haveto be considered for each antenna. The beam forming is calculated thenby a numerical approximation, wherein for each communication partner towhich a beam shall be directed, a separate calculation is needed. Thesecalculations are carried out in the channel cards 9 of the radioequipment control unit 5.

The beam forming can be a two-dimensional beam forming, wherein the beamis focused only in a horizontal plane. It is also possible to carry outa three dimensional beam forming, when the beam is focused both in thehorizontal and in the vertical plane.

Preferably, the calculation of the phase differences and the amplitudesare carried out in the base band. The thus calculated base band signalcan directly be shifted into a radio frequency signal. As the mixingprocess is a multiplication with e^(jωτ), the phase and amplitudeinformation is also provided in the radio frequency signal. Preferablythe spectrum of all channels is completely calculated in the base band.

Alternatively it is also possible to calculate the phase differences inthe radio frequency signal. However, this requires analog phase shifter.PIN-diodes together with delay lines can be used as analog phaseshifters.

The MIMO mode and the beam forming mode are only relevant with respectto emitting antenna signals. Antenna signals of multiple sources aresuper-imposed so that each antenna 2 receives the signals of differentsignal sources. The received antenna signals are digitized by means ofan analog-to digital converter (ADC) of the RF-base band module 18 andare transformed to the base band. The base band data are transmitted tothe radio equipment control unit 5. In the radio equipment control unit5 the base band data are decoded. The decoded digital data can bediscriminated e.g. with respect to the source. The signals of a certainsource received by different antennas 2 can be further investigated.Particularly, it is possible to carry out a beam selection, according towhich it is determined from which direction the corresponding signalsare received. Such a beam selection can be done by adding or multiplyingthe signals, particularly if one of the signals is phase-shifted. Forexample, phase-shifting one signal about a certain amount and adding itto a not shifted signal received by a different antenna 2 can result ina destructive interference with no or a very small signal level. Fromthis phase difference the receiving angle can be deduced. For scanning acertain range two signals of neigbouring antennas 2 are stepwise phaseshifted to each other by incremental steps, wherein for each step thesum or difference of the two signals is calculated. The phase at whichthe sum or difference provides a maximum or minimum is determined. Fromthis phase the direction of the signal (DoA: Direction of Arrival) canbe calculated.

The base band data are sent in data containers via the antennainterconnect 6 from the control RF-base band unit 14 to the radioequipment control unit 5. Before putting the base band data into thecontainers, the amount of base band data is reduced by a lossless datacompression and/or a lossy data reduction process. A suitable losslessdata compression is e.g. the Lempel/ZiV LZ 77 algorithm which have asignificant data reduction because of the high similarity of thereceived individual antenna signals. In an lossy compression however,not sensible, relevant, redundant or similar information, respectively,or disturbing information can be eliminated. Disturbing information ise.g. noise or not used channels. As usually with involvement of theradio equipment control unit 5 the number of users, the activity and thetransmitting/receiving situation is known, a plurality of data can beeliminated or deleted, wherein the useful data or information ismaintained.

This kind of data compression is particularly efficient in combinationwith an AAS having a distributed architecture of the radio equipment 3,which means that to each antenna 2 an individual transceiver module 12is assigned. Thereby, the digital signals exchanged with antenna modules23 are very similar so that the compression is very efficient.

The sensitivity of the whole system can be increased if the data of theindividual antennas 2 are super-imposed. Such a super position can becarried out by adding of data received by different antennas 2. As thedata of the different antennas 2 are correlated to each other but notthe noise, the signal to noise-ratio is increased.

Data signals which are transmitted from the radio equipment control unit5 to the antenna modules 23 consist usually of an information which isto be transmitted to the recipient or communication partner 63, which iscalled message data in the following, and the amplitude/phaseinformation. The message data is identical for all antenna modules 23 inthe beam forming mode. The data signals differ with respect to the phaseand/or amplitude information. Therefore, it is appropriate to send oncethe message data and to send separately from the message data theamplitude and/or phase information for the several antenna modules 23.The message data are e.g. distributed by the data network connecting theantenna modules 23. This results in a significant reduction of the dataamount which has to be transmitted via the antenna interconnect 6.

A further possibility to reduce the data amount is to predict thetrajectory of a communication partner 63 and to calculate thecorresponding phase shifts and/or amplitude shifts for directing a radiosignal by means of beamforming to the communication partner 63. Thecalculation of the phase shifts and/or amplitude shifts can be carriedout in the radio equipment 3 so that the message data have to betransmitted only once from the radio equipment control unit 5 to theradio equipment 3. In certain time intervals the accuracy of theprediction is checked and if necessary corrected.

This method is particularly efficient if the communication partner 63 isa mobile phone installed in a driven car which moves usually with arather constant speed and direction. The direction is usually known bythe coordinates of the street.

The reduction of the data amount which is to be transmitted between theradio equipment 3 and the radio equipment control unit 5 improves thequality of the whole system significantly, because with the same amountof data as transmitted between the radio equipment 3 and the radioequipment control unit 5 in the ordinary active antenna systems, a muchhigher quality of beam forming or beam selection is possible and muchmore complex antenna signals are possible. The similarity of the data ofdifferent channels allow a reduction to about 30-40% of the originaldata amount without reducing the quality of the data.

According to a further embodiment of the present invention thetransceiver module 12 comprises means for calibrating each transceivermodule 12 separately with respect to phase and amplitude of thetransmitted antenna signals. The Tx-transmission chip 16 comprises atransmission path 30 and a reference path 31 (FIG. 8a ). Thetransmission path 30 comprises a digital predistortion unit 32 (DPD),one or more intermediate frequency-up converter (f_(IF)) 33, adigital-to-analog converter 34 (DAC), a channel frequency-up converter35 and a power amplifier 36 (PA). The digital predistortion unit 32reduces any non-linearity of the power amplifier 36. The intermediatefrequency-up converter (f_(IF)) 33 can be a sample rate converter ordigital upconverter (DUC) for converting the digital base band signal onthe base band frequency onto a digital intermediary frequency. Then thechannel frequency-up converter 35 converts the analog intermediaryfrequency signal onto the carrier signal on the carrier frequency.

The reference path 31 comprises an external coupling element 37 which iscoupled to the connection between the duplex filter 15 and the antenna 2for decoupling or branching-off an external calibration feedback signalfrom the channel signal which is transmitted from the duplex filter 15to the antenna 2. The external coupling element 37 is e.g. a Langecoupler or a directional coupler. Further, the reference path 31comprises an intermediate frequency-down converter 38, ananalog-to-digital converter 39 (ADC) and a base band (digital) downconverter (DDC) 40. The analog external calibration feedback signalhaving the carrier frequency or samples of frequencies in the broadbandis converted onto an intermediate frequency by the intermediatefrequency-down converter 38. Then the base band down converter 40converts the digital intermediate frequency signal onto the base bandfrequency. The frequencies of the external calibration signal arepreferably in gaps between the regularly used frequencies for notdisturbing the data transmission. Thus, the reference path 31 providesan external calibration feedback signal branched-off from the channelsignal supplied to the antenna 2, wherein the external calibrationfeedback signal is down-converted to the base band frequency.

The calibration module 20 comprises a digital signal processor 41 (DSP)and a calibration unit 42. The digital signal processor 41 is connectedto the reference path 31 for receiving the down-converted externalcalibration feedback signal and is also connected to the RF-base bandchip 18 for receiving the base band signal. The digital signal processor41 compares the base band signal and the external calibration feedbacksignal and provides a comparison information to the calibration unit 42.The calibration unit 42 is also connected with the RF-base band chip 18for receiving the base band signal and is connected with theTx-transmission chip 16 for supplying a modified baseband signal to theTx-transmission chip 16. The calibration unit 42 modifies the base bandsignal according to the comparison information so that the differencesbetween the base band signal and the down-converted external calibrationfeedback signal are minimized.

In a preferred embodiment of the present invention an internal couplingelement 43 is additionally provided at the transmission path 30 betweenthe channel frequency-up converter 35 and the duplex filter 15 fordecoupling or branching-off an internal calibration feedback signal(FIG. 8b ). The internal coupling element 43 is connected to a switch 44provided in the reference path 31, which can connect the reference path31 either with the internal coupling element 43 for receiving theinternal calibration feedback signal or with the external couplingelement 37 for receiving the external calibration feedback signal. Theduplex filter 15 has a certain impact on the channel signal whichdepends on different parameters, particularly the temperature. Bycarrying out a calibration on the basis of the internal calibrationfeedback signal and the external calibration feedback signal, thisimpact of the duplex filter 15 can be eliminated. The internalcalibration feedback signal is particularly used for adjusting thedigital predistortion unit 32. This can be achieved by analyzing theharmonics of the channel signal which is output by the power amplifier36. The duplex filter 15 lets only pass the signals in the channel band,so that the harmonics are eliminated. Thus, this information is notavailable in the external calibration feedback signal. Such acalibration means comprising an internal calibration feedback signal andan external calibration feedback signal allows to compensate forfluctuations of low cost duplex filters 15 which are preferred in anactive antenna system, because each antenna module 23 needs such aduplex filter 15.

In the above embodiment the calibration feedback signals aredown-converted by means of an analog intermediate frequency downconverter 38. This intermediate frequency down converter 38 can beprovided as a mixer. Instead of such a mixer also other non-linearelements such as a diode or an analog-to-digital converter with a lowresolution and low scanning frequency (Nyquist-converter) may be usedinstead of the intermediate frequency down mixer 38.

According to a further embodiment of the present invention thetransceiver module 12 comprises also means for calibrating eachtransceiver module 12 separately with respect to phase and amplitude ofthe received antenna signals. The transceiver module 12 comprises areceiving path 45 and a reference path 46 (FIG. 9a ). The receiving path45 extends from the duplex filter 15 to the RF-base band chip 18 via theRx-receiving chip 17 and the calibration module 20. The Rx-receivingchip 17 comprises a receiving path 45, a low noise amplifier 47, a bandfilter 48, an intermediate frequency-down converter 49, an intermediatefrequency band or low pass filter 50, a base band down converter 51 andan analog-to-digital converter 52. These elements are provided foramplifying the received antenna signal and converting it down to thebase band frequency. Any other known means for receiving anddown-converting a high frequency signal to a base band signal can alsobe used (e.g. direct conversion). The output of the Rx-receiving chip 17is connected with the calibration unit 42 of the calibration module 20and with the digital signal processor 41. The reference path 46 extendsfrom the digital signal processor 41 via the Rx-receiving chip 17 to anexternal coupling element 53 which is provided at the connection betweenthe duplex filter 15 and the antenna 2 for coupling a referenceinjection signal on the carrier frequency into the connection betweenthe duplex filter 15 and the antenna 2. The reference injection signalis generated in the digital signal processor 41 of the calibrationmodule 20. The Rx-receiving chip 17 comprises a reference path 46 havinga channel frequency-up converter 54 and a digital-to-analog converter 55(DAC).

The digital signal processor 41 generates the reference injection signalas a digital signal in the base band frequency. This reference injectionsignal is converted-up to the channel frequency (receiving bandfrequency) or carrier frequency, respectively, by the channelfrequencyup converter 54 and converted into an analog signal by thedigital-to-analog converter 55. The analog reference injection signal onthe carrier frequency is coupled by means of the external couplingelement 53 to the input side of the duplex filter 15. The referenceinjection signal is then amplified, filtered and down-converted to thebase band frequency by the elements along the receiving path 45. Thereceived reference injection signal, now on the base band, is thenforwarded to the digital signal processor 41 where it is compared to theoriginal reference injection signal generated by the digital signalprocessor 41. The digital signal processor 41 provides comparisoninformation to the calibration unit 42 which modifies the base bandsignal received from the RF-base band chip 18 so that the differencesbetween the modified received signal and the generated referenceinjection signal are minimized.

In a preferred embodiment of the present invention there is anadditional internal coupling element 56 in the receiving path 45 betweenthe duplex filter 15 and the low noise amplifier 47 (FIG. 9b ). Thisinternal coupling element 56 is connected by a switch 57 to thereference path 46. With the switch 57 the reference injection signal canbe either applied to the internal coupling element 56 or to the externalcoupling element 53. Thus, the influence of the duplex filter 15 can beeliminated by calibrating the receiving path 45 of the transceivermodule 12 by the reference injection signal applied to the internalcoupling element 56 and by the reference injection signal applied to theexternal coupling element 53.

By the above described embodiments of the present invention it isobvious that each antenna module 23 can be calibrated in itself withoutthe need to transfer any calibration signal to the outside of therespective antenna module 23. This provides for a more exact calibrationthan in ordinary active antenna systems since calibration connections tothe outside comprise delays and can be exposed to disturbances.

The antenna modules 23 receive via the antenna interconnect 6 or theintermediate cable 27 data which are to be transmitted. The antennainterconnect 6 and the connections via the intermediate cables 27 arepreferably a serial digital bus so that the antenna modules 23 can beconnected in any kind of topology such as daisy chain, ring and treetopology and combinations thereof. At least one antenna module 23 isconnected with the radio equipment control unit 5. The distance(s)between one or more antenna modules 23 and the radio equipment controlunit 5 and the distances between connected antenna modules 23 candiffer, so that the several antenna modules 23 receive the data withdifferent delays and phases. The calibration module 20 of the controlRF-base band unit 14 is embodied for delaying the output of the dataaccording predetermined delaying coefficients so that all antennamodules 23 are synchronized. The delaying coefficients are calculated inthe DSPs 41.

The antenna modules 23 can be configured in a daisy-chain (FIG. 10a, 10b) wherein the radio equipment control unit 5 forms a data source whichis sending to the several antenna modules 23 data signals via an antennacable 10 of the antenna interconnect 6 and the intermediate cables 27.FIG. 10a shows only two antenna modules 23, but any number of antennamodules 23 can be arranged in the daisy-chain. The data signals are sentby the radio equipment control unit 5 via the antenna cable 10 andreceived by the first antenna module 23 in the daisy-chain which isdirectly connected to the radio equipment control unit 5, that is theantenna module 23 nearest to the radio equipment control unit 5. Thenthe data signals travel through the daisy-chained antenna modules 23 viathe intermediate cables 27 to the antenna module 23 which is mostdistant from the radio equipment control unit 5.

This most distant antenna module 23 is embodied so that a predeterminedsynchronization signal, which is transmitted from the radio equipmentcontrol unit 5 via the antenna cable 10 and the intermediate cables 27to the antenna modules 23, is returned from the most distant antennamodule 23 via the same intermediate cables 27 and the same antenna cable10 to the radio equipment control unit 5. Since the synchronizationsignal travels from the radio equipment control unit 5 through theantenna cable 10 and all intermediate cables 27 in the daisy-chain tothe most distant antenna module 23 (forward direction) and then the wayback to the radio equipment control unit 5 again through the sameintermediate cables 27 and the antenna cable 10 (reverse direction), thetime interval between transmitting the synchronization signal from andreceiving it at the radio equipment control unit 5 results from twicethe summarized delay times of the antenna cable 10, of all intermediatecables 27 and of all antenna modules 23 in the daisy-chain. Similarly,this calculation also applies to each specific antenna module 23 in thedaisy-chain, but the time interval between receiving the synchronizationsignal in forward direction and receiving the same synchronizationsignal in reverse direction results from twice the summarized delaytimes of all intermediate cables 27 connecting the respective antennamodule 23 with its successive antenna modules 23 in forward directionand of the successive antenna modules 23.

The calibration modules 20 are detecting the time when thesynchronization signal passes the respective antenna module 23 on theway from the radio equipment control unit 5 to the most distant antennamodule 23 and on its way back to the radio equipment control unit 5. Thedifference of these two time data is calculated and divided in half. Theresulting time value form the delay time Δt.

FIG. 10b shows an example of a configuration comprising four antennamodules 23 M1, M2, M3, M4, configured in a daisy-chain. The timeintervals for transmitting the data signals between the radio equipmentcontrol unit 5 and the first antenna module M1 is t1, between the firstantenna module m1 and the second antenna module M2 is t2, between thesecond antenna module M2 and the third antenna module M3 is t3 andbetween the third antenna module M3 and the fourth antenna module M4 ist4. So the complete duration for transmitting the data signals from theradio equipment control unit 5 (data source) to the most distant antennamodule M4 is t1+t2+t3+t4. FIG. 10b comprises a timing diagram of thesignals, wherein the signals when they were received from the antennamodules on the way from the source to the most distant antenna module M4(forward direction), are designated with ST, the signals which arereceived by the respective modules on the return way (reverse direction)are designated with SR and for delayed signals are designated with SD.

The antenna module M1 receives a signal ST at the time t1. This modulereceives the returning signal SR at the time t1+t2+t3+t4+t4+t3+t2. Thetime difference between the transmitted signal ST and the returnedsignal SR is 2 (t2+t3+t4). This difference is divided in half forcalculating a delay time Δt which is for the first antenna module M1t2+t3+t4. The delayed signal SD is the original signal ST delayed by Δt.Thus, the signal SD is provided in the calibration module 20 to thedigital signal processor 41 at the time t1+t2+t3+t4.

As it can be seen from FIG. 10b by calculating for each antenna module23 an individual delay time Δt and delaying the incoming signal ST bythis delay time Δt all antenna modules 23 are processing the delayedsignals SD exactly at the same time (t1+t2+t3+t4).

The synchronization signal can be any data signal. It is also possibleto use a special synchronization signal or a clocking signal which donot contain any data as synchronization signal. Preferably, thesynchronization is carried out at the booting of the active antennasystem and is repeated at certain intervals.

The antenna modules 23 can also be arranged in a tree topology so thatcertain antenna modules 23 are connected to several further antennamodules to which they are transmitting the data signals. An antennamodule 23 which is connected to several antenna modules 23 is calledbranching antenna module 23, because the signal path starting at thedata source is branching at these antenna modules 23 to the furtherantenna modules 23. FIG. 11a shows schematically a functional blockdiagram of a branching antenna module 23 being part of a tree topologycomprising a radio equipment control unit 5 as data source, thebranching antenna module 23/0 and further antenna modules 23/1-23/n. Ineach branch one or more antenna modules 23 can be provided, wherein theantenna modules 23 can be configured in these branches in a daisy-chainconfiguration or a tree configuration or a combined daisy-chain or treeconfiguration. In each branch the most distant antenna module 23 isconfigured for returning a synchronization signal back to the datasource. In the branching antenna module 23/0 the calibration module 20is embodied for detecting the time when the synchronization signal ispassing this branching antenna module 23 from the data source to theantenna modules 23 in the respective branch and the corresponding timeof the returning synchronization signal. The difference of the twotiming values is calculated and divided in half so that for each brancha delay value t1, t2, . . . , tn is calculated.

As each branch can comprise several branches, the synchronization signalcan be returned on a certain branch several times, once for eachsub-branch. So it can be that for each branch several timing values forthe returning synchronization signal are received. The calculation ofthe delay times t1, t2, . . . , tn is based on the latest timing valuefor the returning synchronization signal which corresponds to thelongest transmission path in each branch.

In the branching antenna module 23 the largest delay time t1, t2, . . ., tn of the individual branches is selected as delay interval At of thisbranching antenna module 23/0. With this delay interval data Δt theincoming data signals are delayed in the calibration module 20 beforethey are further processed by the digital signal processor 41.

Furthermore, the branching antenna module 23 delays the data signals foreach individual branch by the delay interval data Δt minus theindividual delay time t1, t2, . . . , tn of the respective branch. Thus,the data signals are less delayed in long branches than in shortbranches.

FIG. 11b shows an example of a mixed tree- and daisy-chain-topology.This active antenna system comprises the radio equipment control unit 5as data source, two branching antenna modules M1, M4 and further antennamodules M2, M3, M5, M6, and M7.

The longest signal path extends from the radio equipment control unit 5(or any other reference point like hub, switch . . . ) via the antennamodule M1, M4 to the most distant antenna module M6. The antenna modulesM3, M5, M6, and M7 are returning the synchronization signals.

The branching antenna module M1 receives four times a returningsynchronization signal, wherein the delay interval Δt is calculated onthe basis of the returning signal of the antenna module M6, because thisis the most distant antenna module in this configuration. Thus, thedelay interval Δt in the branching antenna module M1 is t4+t6.

The delay to the branch containing the antenna modules M2, M3 isΔt−(t2+t3). So the data signal reaches the antenna module M2 at a timewhich is delayed from starting the signal at the source 5 by t1 and t2due to the runtime between the source 5, the antenna module M1 and theantenna module M2 and by the delay Δt−(t2+t3).

In the antenna module M2 the data signal is delayed by Δt=t3 which is adelay according to the daisy-chain of M2 and M3.

The branching antenna module M1 does not delay the data signals to thefurther branching antenna module M4, because this branch provides themaximum delay in the branching antenna module M1, thus Δt−tn is 0(tn=t4+t6). In the branching module M4 the delay interval Δt′ is t6.Thus, the data signals to the antenna module M5 are delayed by Δt′−t5,the data signals to the antenna module M7 are delayed by Δt′−t7 and thedata signals to the antenna module M6 are delayed by Δt′−t6 which is 0.

Thus, in all data modules M1-M7 the processing of the delayed datasignals SD starts simultaneously at the time t1+t4+t6.

The antenna modules 23 are connected by antenna connects 6 orintermediate cables 27. All antenna modules 23 are automaticallysynchronized and there is no need for exactly define the length for theantenna interconnect(s) 6 and the intermediate cables 27. The antennainterconnects 6 should have substantially a similar length to that thedelay times for the individual branches are minimized. Thissynchronization mechanism facilitates the design of the antennainterconnects 6 and also of the intermediate cables 27 significantly.

In the above-described embodiments of the synchronization mechanism theradio equipment control unit 5 forms the data source of the activeantenna system. The data source can be also one of the antenna modules23, wherein the other antenna modules 23 are synchronized with respectto the antenna module 23 which forms the data source. In such a case thecalibration module 20 of the “data source” generates the synchronizationsignal.

The delays of all antenna modules 23 can be calculated in the datasource or it is also possible, that each antenna module 23 iscalculating the respective delay by measuring the time differencebetween the synchronization signal and the returning synchronizationsignal.

According to another embodiment it is also possible that each antennamodule 23 is measuring the delay(s) to the neighboring antenna module(s)23. On the basis of these measured delays the delay coefficients for therespective antenna module 23 is determined. In such a method thedetermination of the delays has to be arbitrated, e.g. by determiningone antenna module 23 as master.

According to a further embodiment the determination of the individualdelay times Δt is carried out by a method as described in the followingwith reference to FIG. 11b . For using this method the synchronizationsignals comprise headers with data fields, which can be read and writtenby each antenna module 23 as well as by the radio equipment control unit5. In FIG. 11b the branch which comprises the longest delay is thebranch from the radio equipment control unit 5, passing antenna moduleM1, passing antenna module M4, and ending at antenna module M6, thus thelongest delay is t1+t4+t6. Each antenna module 23 is embodied to letonly pass the returning synchronization signal comprising the longesttime interval to its predecessor, such as another preceding antennamodule 23 or the preceding radio equipment control unit 5. Thissynchronization signal is the latest received returning synchronizationsignal.

In a first step the radio equipment control unit 5 transmits a firstsynchronization signal to the antenna modules 23. The time differencesbetween the synchronization signal and the returning synchronizationsignal is measured by the radio equipment control unit 5 and by eachantenna module 23, for each connected branch separately. Then themeasured time differences are divided by 2 in the radio equipmentcontrol unit 5 and in each antenna module 23. For example, antennamodule M4 calculates three time differences t5, t6 and t7 for threebranches and, since the returning synchronization signal received fromantenna module M6 is the latest, only this synchronization signal ispassed from antenna module M4 to antenna module M1. Another example isantenna module M1, which calculates two time differences t2+t3 and t4+t6for two branches and lets only pass the returning synchronization signalreceived from antenna module M4 to the radio equipment control unit 5.The radio equipment control unit 5 measures all time differences of allreturning synchronization signals on all branches and calculates therespective time differences. Further, the radio equipment control unit 5determines the longest time difference, which is the time difference ofthe latest returning synchronization signal from all branches, as“maximum delay time” of the active antenna system. Since FIG. 11b onlyshows one branch being connected to the radio equipment control unit 5,the radio equipment control unit 5 calculates only one time differencet1+t4+t6, which is also the “maximum delay time”.

In a second step the radio equipment control unit 5 transmits a secondsynchronization signal to the antenna modules 23. The radio equipmentcontrol unit 5 writes the “maximum delay time” to a data field (“maxdelay time”) of the synchronization signal and the calculated timedifference of each connected branch to another data field (“branch delaytime”) of the synchronization signal, which is then transmitted to therespective branch. This means, that the succeeding antenna modules 23 ofevery branch receive a second synchronization signal comprising the“maximum delay time”, which is the same for all branches, and comprisingthe “branch delay time”, which is a unique value for each branch andwhich represents its own time difference or delay.

In a third step, each antenna module 23 receives the secondsynchronization signal, extracts the “maximum delay time” and the unique“branch delay time”. Then, each antenna module 23 writes the valuecalculated before for each connected branch to the “branch delay time”and passes the modified synchronization signal to the respective branch.This means, that the “maximum delay time” is left untouched and the“branch delay time” is set to the unique value which was calculated inthe first step for the respective branch. For example, the antennamodule M1 writes the value of t2+t3 to “branch delay time” of the secondsynchronization signal and passes the modified signal to antenna moduleM2. Further the antenna module M1 writes the value of t4+t6 to “branchdelay time” of the second synchronization signal and passes the modifiedsignal to antenna module M4. Then, each antenna module 23 calculates thedifference between the extracted “branch delay time” and the largest“branch delay time” of its succeeding branches, which was calculated inthe first step. For example, the antenna module M1 calculates(t1+t4+t6)−(t4+t6)=t1, the antenna module M4 calculates (t4+t6)−(t6)=t4.Thus, each antenna module 23 calculates the time difference of theconnection to its preceding antenna module 23 (“preceding delay”).

In a forth step, the radio equipment control unit 5 transmits a thirdsynchronization signal to each connected branch which comprises anotherdata field (“sum delay”) with the value “0”. While passing thesynchronization signal, each antenna module 23 extracts the value fromthe data field “sum delay” of the synchronization signal, sums theextracted value with the “preceding delay” and writes the result to thedata field “sum delay”. For example, the antenna module M1 extracts “0”from the data field “sum delay”, calculates 0+t1 and writes t1 to thedata field “sum delay”, while the antenna module M4 extracts t1 from thedata field “sum delay”, calculates t1+t4 and writes this value to thedata field “sum delay”. Thus, each antenna module 23 now knows the sumof the delays to itself in its branch.

In a fifth and last step each antenna module 23 subtracts from the“maximum delay time” the “sum delay” and applies this individual delaytime Δt to a delay line which delays the antenna signal beingtransmitted and/or received. For example antenna module M1 calculates(t1+t4+t6)−t1=t4+t6, while antenna module M4 calculates(t1+t4+t6)−(t1+t4)=t6. Thus, all antenna modules 23 transmit or receivethe antenna signals at the same point of time.

If timing problems, which can cause errors in measurements, arise byreading from and writing to the data fields in the synchronizationsignal, while the synchronization signal is passed through an antennamodule 23, then the radio equipment control unit 5 can distribute thesetiming relevant steps to several sub steps. For this a furthersynchronization signal is transmitted which only purpose is to providethe data fields for writing.

In the above description of the method for determination of theindividual delay times Δt the delay each antenna module 23 hasinternally is omitted for better understandability.

According to a further embodiment the determination of the calibrationcoefficients is carried out for all or the predetermined antenna modules23 in the radio equipment control unit 5 and the delay is also carriedout in the radio equipment control unit 5. Thus all or the predeterminedantenna modules 23 do not delay the antenna signals. In such a case thecalibration coefficients can be combined with the delays and phase shiftaccording to the beamforming or other corrections.

The above-described synchronization mechanism can also be used forsynchronizing antenna modules 23 being configured in a ring-topology ora ring-topology mixed with a daisy-chain-topology and/or atree-topology. On the point of view of the data source, a ring-topologyforms two branches, one in clockwise direction and one incounter-clockwise direction. These two directions in the ring aretreated as separate branches. Thus, a ring-topology can be lead to atree-topology having two branches.

The clock signals in each antenna modules should be very precise. Adeviation of about 14 ps means a phase-shift of 5° for a signal having afrequency of 1 GHz. The clock signals should clock to about 1/10 of thebit frequency. The byte clock speed should be e.g. about 983 MHz (for ananticipated 10 Gbps signal). According to one embodiment such a clocksignal is e.g. centrally generated and transmitted by a separateconduction path having the same length to each module and they arestabilized by a high-precision oscillator and a PLL, so that the clocksignal provides a ps-accuracy. This high-precision clock signal can beused for controlling digital-to-analog converters and analog-to-digitalconverters. FIGS. 12a and 12b show an example of the distribution ofsuch a clock signal in an antenna module 23.

According to another embodiment the above described mechanism forsynchronizing the data signals can also be used for synchronizing clocksignals. Each antenna module comprises a separate clock havingpreferable a high precision oscillator and a PLL for stabilizing theclock signal. The individual clocks of the antenna modules cancommunicate with each other via the data bus. During configuration oneof these clocks forms a master clock. This master clock sends a clocksignal to one or more further clocks, which are slave clocks. The clocksignal comprises a time stamp. The slave clock receives the clock signalfrom the master clock together with the time stamp and calibrates itsclock signal according to the time difference between the time stamp andthe time when the slave clock received the clock signal from the masterclock. Then the slave clock returns its clock signal back to the master,so that the master can verify whether the calibration of the slave iscorrect or not. In case of a deviation the calibration is repeated.

Once a slave clock is calibrated it can act as master clock forcalibrating clocks of further antenna modules.

The clock of the radio equipment control unit 5 can be used as masterclock for clocks a certain group of antenna modules 23. After havingcalibrated these clocks then these clocks can be used as master clocksfor further clocks of further antenna modules.

In the following a high performance amplifier for amplifying the highfrequency signals having a high peak-to-average ratios output by adigital-to-analog converter is explained.

Usually a digital-to-analog converter generates internally a currentsignal for each bit of the digital signal, wherein each internal signalcomprises either the current 0 or the current 2^(n)·I₀, wherein I₀ isthe current of the 0-bit. These several currents are summarized in asummarizing node for providing the complete analog current signal.

In the present embodiment (FIG. 13a ) several amplifiers 58 are providedfor amplifying an output of a single bit of the digital-to-analogconverter or a combined output of a limited number of bits. In theembodiment according to FIG. 13a the digital-to-analog converter 55comprises as an example a 6 bit output, wherein only the bits 0, 1, 2,and 3 are summarized on a summarizing node 59 and the further bits 4 and5 are individually amplified by separate amplifiers 58/2 and 58/3. Theoutputs of the amplifiers 58/1, 58/2, 58/3 are combined on a furthersummarizing node 60. The amplification characteristic of the amplifiers58/1, 58/2 and 58/3 is adapted to the individual current levels of thedifferent outputs of the digital-to-analog converter 55. Particularly,it is preferable to provide a different amplification characteristic forhigh current levels in comparison to low current levels. Particularly,the amplifier 58/1 for the lower bits needs a smaller maximum outputcurrent in comparison to the amplifiers 58/2 and 58/3 for the higherbits.

By amplifying the current outputs of the bits or group of bits beforethe current of all bits are combined in a final summarizing nodeimproves the linearity and the response characteristic of theTx-transmission chip 16.

According to a further embodiment the current outputs of each bit of thedigital-to-analog converter is amplified by a separate power amplifier58/1-58/6. For each bit a certain current source 61 is arrangedproviding a current with the level 2n·I0, wherein n is the number of thebit. The current source 61 is connected by a switch 62 to thecorresponding power amplifier 58/1-58/6. If the switch 62 is closed,then the corresponding bit is on and if the switch 62 is open, then thecorresponding bit is off. The currents of the respective bits areamplified by the individual amplifiers 58/1-58/6 which are adapted tothe corresponding current level. The individual amplifiers 58/1-58/6 areadapted to the corresponding current level in that the area ofcorresponding transistors is adapted to the current level or severaltransistors are connected in parallel according to the current level.Such an embodiment comprises identical amplification paths for each bitso that the delay for each bit is identical. This provides a veryhomogenous integration into the Tx-transmission chip 16. The individualamplified currents are summarized at the summarizing node 60. Thecompletely summarized signal forms the antenna signal which is appliedto the antenna 2 without further amplification.

With such an amplification unit individual bits or groups of bits of theoutput of the digital-to-analog converter are amplified separately andthen summarized by a summarizing node. There is no need for a furtheramplification for applying the complete antenna signal to the antenna 2.Such an amplification unit can be realized by low costs, with a highlinearity and with a high efficiency. Furthermore, such an amplificationunit can be embodied completely as an integrated circuit insemiconductor technology. (wenn die Ausgangsleistung nicht zu hock ist,sonst besser diskret)

These high performance amplifiers can be embodied on a chip either withdiscrete elements and/or with integrated elements. These highperformance amplifiers can be used in any multi carrier system havinghigh peak-to-average ratio signals. Furthermore, an impedance convertercan be connected with the output of the summarizing node. Further signalprocessing elements can be provided between the power amplifiers and thesummarizing node.

The above described active antenna systems 1 can be used for a securewireless communication with a communication partner 63.

In the following the steps for establishing a communication connectionbetween the active antenna system 1 and a communication partner 63(FIG. 1) are explained:

1. The establishing of a communication connection starts with scanningfor a communication partner 63. The active antenna system 1 can emit abeam-shaped scanning signal which direction is permanently changed sothat the scanning signal is scanning during certain time intervals acell 64 for new potential communication partners. If the communicationpartner 63 receives such a scanning signal, then it sends apredetermined scanning reply signal which is received by the activeantenna system. The scanning reply signal can comprise an identificationcode such as IMEI and/or the telephone number, so that the activeantenna system can identify a communication partner 63.

Alternatively the scanning for a communication partner can be triggeredby the communication partner 63 by sending a contact signal. The sendingof this contact signal is initiated by the communication partner 63without receiving any signal from the active antenna system 1. Theactive antenna system 1 sends a contact reply signal after receivingsuch a contact signal to the communication partner. By the exchange ofthe contact signal and the contact reply signal the communicationpartner 63 and the active antenna system 1 can identify themselves toeach other.

In case that the communication partner 63 is moving connectioninformation from one cell 64 can be transmitted to the active antennasystem of a neighboring cell to which the communication partner 63 ismoving, so that the active antenna system 1 of the neighboring cell isalready expecting the communication partner 63. Preferably, theestimated position of the communication partner 63 is transmitted fromone cell 64 to the other cell 64, so that the active antenna system 1 inthe second cell can search at a certain region in the cell for thearriving communication partner 63.

The scanning signal, the scanning reply signal, the contact signal, andthe contact reply signal are only provided for getting into contactbetween the communication partner 63 and the active antenna system 1 andnot for exchanging any information besides the optional identificationinformation (IMEI and/or the telephone number).

2. After the active antenna system 1 has recognized a communicationpartner 63 it estimates the direction of arrival (DOA) from the receivedscanning reply signal or contact signal. Eventually a further exchangeof scanning signals and scanning reply signals is initiated forestimating the direction of arrival and/or for measuring physicalparameters of the radio channels of each antenna 2 of the active antennasystem 1.

The direction of arrival can be estimated by measuring the phasedifferences of the scanning reply signal or contact signal received bythe several antennas 2. From the results of the phase measurements thecorresponding DOA-parameters Φ₁, . . . , Φ_(N) can be derived. There arealso other methods known for estimating the direction of arrival (DoA)which can also discriminate reflected signals and disturbing signalswhich use additional information, particularly the completely encodedand descrambled signals. These further methods for estimating thedirection of arrival (DoA) can be also used.

The physical parameters of the radio channels of the individual antennas2 can be measured by means of a channel impulse response (CIR).Different methods are known for measuring the channel impulse response.This can be done by applying a Dirac-impulse or a Kronecker-Δ in adiscrete-time system, detecting a step response or applying a broadbandnoise to the channels. Any linear, time-invariant (LTI) system iscompletely characterized by its impulse response. The impulse responsecan be converted into a transfer function by the Laplace transform. Withsuch a method the CIR-coefficient h_(n), . . . , h_(N) can bedetermined. Thus, the CIR-coefficient of each radio channel and thedirection of each radio channel are known. These CIR-coefficients arephysical parameters which are varying in time and place. Thus theseparameters are very individual numbers which cannot be reproduced bysomeone else besides copying the original data.

3. The active antenna system 1 is calculating beam-forming parametersa₁₀₁₁, . . . , a_(ΦN) for each radio channel of each antenna 2 which isdirected to the detected communication partner 63.

4. Predistortion factors w₁, . . . , w_(N) are calculated forcompensating the distortion in the individual radio channels. Thepredistortion factors are weighting factors for inversely reproducingthe radio channels (w₁, . . . , w_(N)↔h⁻¹ ₁, . . . , h⁻¹ _(N)). Theseweighting factors form together with the beam-forming parameters abeam-forming matrix.

5. A signal s(t) containing certain data which are to be transmitted issub-divided in several partial signals d(t), wherein the partial signalsd(t) are distributed onto the N radio-channels. This can be expressed bythe following formula:

$\begin{matrix}{{s(t)} = {{d(t)}{\sum\limits_{\forall n}{w_{n}a_{\Phi_{n}}}}}} & (5)\end{matrix}$

6. The signal s(t) is transmitted via the several radio channels fromthe active antenna system 1 to the communication partner 63. Thecommunication partner 63 receives the signal s(t) or the partial signalsd(t) from all directions and superimposes the received signals d(t). Asthe partial signals d(t) are transmitted via several radio channels andby means of a directed or beamformed radio signal, the superpositiondepends very much from the location of the communication partner 63. Inother words, only if the communication partner 63 is located at thecorrect place which corresponds to the direction which was estimated bythe active antenna system 1 in the above described step 2 then acomplete superposition of the signal s(t) is possible. With this methodit can be secured, that other potential communication partners in thecell 64 cannot receive the complete signal s(t) even if they receive allpartial signals d(t), because at another place in the cell the partialsignals d(t) have a different phase difference and/or runtime and/oramplitude to each other so that the superposition is substantiallydifferent to the intended superposition. This makes it difficult forother potential communication partners to listen to the communicationbetween the active antenna system 1 and the intended communicationpartner 63. Therefore, we call this method spatial encryption. Thespatial encryption is based on the provision of several radio channels(at least two, preferably four or more radio channels) and thebeamforming of the radio signals.

After having established the data communication between the activeantenna system 1 and the communication partner 63 in step 6, anencryption scheme is determined.

After determining the encryption scheme the communication partner 63 canidentify itself by the active antenna system 1 by transmitting anidentification code such as an IMEI or a telephone number, if such anidentification was not yet carried out in step 1.

The encryption scheme can be any ordinary encryption scheme such as RSAor AES (Advanced Encryption Standard) which is negotiated between theactive antenna system 1 and the communication partner 63 by means of theestablished data communication.

This spatial encryption method can be used with each known activeantenna system which has beamforming capabilities.

Alternatively or additionally a measurement of unique data or parametersof the radio channel(s) can be used for determining the encryptionscheme. Particularly, the measurement of the channel impulse responseand the calculation of the CIR-coefficients (as described above) can beused for determining the encryption scheme. The channel impulse responseis a very specific function which describes the physical properties ofthe individual radio channels.

A channel impulse response can be measured by the active antenna system1 and the corresponding CIR-coefficient can be transmitted via the dataconnection to the communication partner 63. Both, the active antennasystem 1 and the communication partner 63 can use the CIR-coefficientsto encrypt the further signals. The usage of the CIR-coefficient ispredefined in both systems, the active antenna system 1 and thecommunication partner 63. This kind of encryption is particularly usedin combination with the above described spatial encryption, so that noone else can listen to the transmission of the CIR-coefficients. The CIRcoefficients which are used for encryption form a time variant key.

If the radio channels are reciprocal then it is possible that both theactive antenna system 1 and the communication partner 63 are measuringsimultaneously the CIR-coefficient independently of each other, whereinthey receive the identical or substantially identical CIR-coefficients.With such two measurements on both sides of the communication path veryspecific coefficients are generated which must not be transmitted viathe communication path. On the basis of these CIR-coefficients theactive antenna system 1 and the communication partner 63 can encrypt thefurther signals.

If the measurement of the CIR-coefficients was not exact enough, so thatthe CIR-coefficients of the active antenna system 1 differ from the oneof the communication partner 63 then the measurement of theCIR-coefficients has to be repeated until sufficiently coincidingCIR-coefficients are achieved. Such an encryption scheme which is basedon the independent measurements of the same physical parameters of theradio channels can also be used independently of the above describedspatial encryption, because the radio channels are so specific that noone else can achieve these parameters. This is particularly true if theradio channels are beamformed (steps 1.-4. of the above describedmethod). There is no need to distribute the signal onto different radiochannels. In a TDD system (time duplex domain system) this method canalso be used with only one single radio channel for securely determiningan encryption scheme for the active antenna system and the communicationpartner.

A further encryption scheme which can be used additionally oralternatively is to scramble a signal containing the data which are tobe exchanged between the active antenna system and the communicationpartner onto the different radio channels. This is a logicaldistribution of the data onto the different radio channels. The activeantenna system 1 and the communication partner have to negotiate how thedata are to be scrambled on the different radio channels. Preferably,such negotiation is repeated regularly, so that the scrambling scheme isregularly changed. The intervals for changing the scrambling scheme arepreferably in the range of 0.1 s to 10 s. Such a dynamic scrambling ofthe data onto the different data paths or radio channels, respectively,is highly secure, because a listener of the different radio channels isnot able to read and even combine the snippets of signals or data.Preferably antennas of different cells are used for this method, becausethese antennas are completely uncorrelated.

Preferably the above encryption schemes are combined using

-   -   spatial encryption, wherein the antenna signals are physically        distributed as partial signals d(t) onto the different radio        channels,    -   the encryption scheme is based on physical parameters of the        radio channel(s), and/or    -   the data which are transmitted are logically distributed onto        the different radio channels statically or dynamically.

In the following a further process for determining the encryption schemeby means of CIR-coefficients is explained, comprising the followingsteps:

a. A reference signal is transmitted by the active antenna system 1several times, wherein each time it is transmitted simultaneously via atleast two radio channels (i.e. at least two antennas 2) (out of n),wherein each time a different combination of the radio channels is used.Thus, the communication partner 63 receives always a reference signalsimultaneously of at least two radio channels.

b. The communication partner 63 measures the CIR-coefficients of thereference signals.

c. The communication partner resends the measured CIR-coefficients tothe active antenna system.

d. The active antenna system calculates the CIR-coefficients for eachsingle radio channel on the basis of the received CIR-coefficients. TheCIR-coefficient of each radio channel is a vector. By measuring thereference signal which is transmitted via at least two radio channels,provides CIR-coefficients which correspond to a vector resulting from anaddition of vectors of the individual radio channels. As only the activeantenna systems know which radio channels were used for transmitting thereference signal, only the active antenna system is able to calculatethe CIR-coefficient for each individual radio channel.

e. By means for the thus received CIR-coefficients for the individualradio channels, the above described spatial encryption is carried out.

As an example it is assumed that three antennas A, B, C are used. Theactive antenna systems 1 transmits the reference signal firstly via theantenna A+B, then via the antenna B+C and then via the antenna A+C. Thecommunication partner measures the CIR-coefficients for each combinationof the two reference signals. The measured CIR-coefficients aretransmitted from the communication partner to the active antenna system.At the active antenna system the CIR-coefficients are calculated bysubtraction of the vectors so that the CIR-coefficients for theindividual radio channels are achieved. These CIR-coefficients are usedfor calculating the weighting factors for the predistortion factors ofthe step 4.

A listener who receives the CIR-coefficients transmitted from thecommunication partner to the active antenna system cannot combine them,because he does not know by which radio channels the reference signalwas transmitted

In the example just three antennas are used. Preferably, more antennassuch as at least five or at least ten antennas are used, wherein thereference signal is simultaneously transmitted by preferably a varyingnumber of antennas such as two, three, four or five antennassimultaneously, so that the number of potential combinations is large.

According to a further embodiment the Rx-receiving chip 17 is replacedby a homodyne receiver 65. Such homodyne receivers are also calleddirect-conversion receiver (DCR), synchrodyne receiver orZero-IF-receiver. The homodyne receiver demodulates the incoming radiosignal using synchronous detection driven by a local oscillator whosefrequency is identical to, or very close, to the carrier frequency ofthe received signal. This is in contrast to standard super-heterodynereceivers where this is accomplished only after an initial conversion toan intermediate frequency. The simplification of performing only asingle frequency conversion reduces the basic circuit complexity butother issues arise, for instance, regarding dynamic range. Thedevelopment of homodyne receivers in low-cost integrated circuits madethis design widely accepted.

In a homodyne receiver an amplitude modulated signal is decomposed intotwo amplitude-modulated sinusoids that are offset in-phase byone-quarter cycle. These amplitude-modulated sinusoids are known asin-phase (I) and quadrature (Q) components.

The homodyne receivers suffer the problem that the in-phase and thequadrature can be offset (IQ-Imbalance) so that mirror frequencies whichare folded into the base band signal are caused. If the offset of thein-phase and the quadrature components is known, then it is possible tocompensate the offset. Thereby, the offset can continuously changeduring operation due to temperature effects and changing otheroperational parameters so that a static compensation of the IQ-imbalancecannot permanently avoid the problem.

The transceiver module 12 of this embodiment comprises a transmissionpath 30 which is substantially identical to the one of theabove-described embodiments comprising an RF-base band chip 18 and aTx-transmission chip 16 (FIG. 14). The homodyne receiver 65 is locatedin the receiving path 45.

An attenuation element 66 is provided in-between the transmission path30 and the receiving path 45 for transmitting a portion of the amplifiedoutput antenna signal u(t) onto the return (standard receive Rx path)path 45. The attenuated signal u_(R)(t) is added as a reference signalin the receiving path 45 to the received antenna signal u_(s)(t).

In the simplest embodiment the circuits of the transmission path 30 andthe receiving path 45 are located so close to each other that thecross-talk of the transmission path 30 to the receiving path 45transmits an attenuated portion of the output antenna signal u_(R)(t)onto the receiving path 45. Other attenuation elements 66 can beprovided, such as e.g. a capacitor or a directional coupler.

The superimposed signals of the reference signal u_(R)(t) and the inputantenna signal u_(s)(t) are applied to the input side of the homodynereceiver 65. The output side of the homodyne receiver 65 is connected toan IQ-compensation module 67 for compensating the IQ-imbalance. Theoutput side of the homodyne receiver 65 is further connected to anIQ-coefficient determination module 68. The IQ-coefficient determinationmodule 68 is provided for determining IQ-imbalance coefficients g₀, g₁,g₂,. . . . The IQ-coefficient determination module 68 is furtherconnected to the input side of the transmission path 30 for receiving acomplex, time-depending amplitude A(t). This complex, time-dependentamplitude A(t) forms a broadband transmission signal. Preferably, thetransmitter and the receiver are run in the FDD (Frequency DivisionDuplex) mode. The transmission Tx-band in the receiving (Rx-band) dousually not overlap after filtering the signals with the duplex filter15. However, it can be assumed that due to non-linear distortions theTx-band emits basically undesired components overlap with the receivingRx-band, or before filtering the signals the transmission band and thereceiving band can overlap. A reference signal can be used whichfrequency lies in the overlap. The desired transmission signal u_(g)(t)having the carrier frequency f_(T)=ω_(T)/2π can be described by:u _(g)(t)={ A (t)·exp(jω _(τ) t)}  (6)

The portions of the transmission signal including non-linear distortionsaround the middle frequency of the receiver (f_(R)=Ω_(R)/2π) are usedfor characterizing the IQ-imbalance of the receiver. A duplex distancef_(D)=f_(T)−f_(R)=ω_(D)/2π is usually small in comparison to the middlefrequency |f_(D)|«f_(τ),f_(D).

The complete transmission signal u(t) can be described withu(t)=Re{(1+a ₃ ·|A (t)|² +a ₅ ·|A (t)|⁴+ . . . )· A (t)·exp(jω _(T)t)}  (7)

wherein the complex coefficient a ₃, a ₅, . . . describes thenon-linearity. A potential frequency dependency of the non-linearitiesis neglected in this formula.

As described above, a portion of the transmission signal is transmittedfrom the transmission path to the receiving path 45 as reference signal,wherein this portion of the transmission signal u_(R)(t) is stronglyattenuated so that the receiving signal is not distorted. Theattenuation factor k is much smaller than 1, |k|«1.

The reference signal at the input side of the homodyne receiver 65 canbe described asu _(R)(t)=Re{k ·(1+a ₃ ·|A (t)|² +a ₅ ·|A (t)|⁴+ . . . )· A (t)·exp(jω_(D) t)·exp(jω _(R) t)}  (8)oru _(R)(t)=Re{B (t)·exp(jω _(R) t)}  (9)withB (t)= k ·(1+a ₃ ·|A (t)|² +a ₅ ·|A (t)|⁴+ . . . )· A (t)·exp(jω _(D) t)  (10)

the complex attenuation factor k can be combined with the complexcoefficients a ₃, a ₅, . . . to the complex coefficients b ₁=k, b ₃=k·a₃, b ₅=k·a ₅, . . . , so that:B (t)=( b ₁ +b ₃ ·|A (t)|² +b ₅ ·|A (t)|⁴+ . . . )· A (t)·exp(jω _(D)t)  (11)

The simplest way to describe the homodyne receiver 65 with IQ-imbalanceis to use the amplitude C(t) instead of the complex, time-dependentamplitude B(t), wherein the amplitude C(t) comprises a small portion ofthe complex conjugate amplitude B*(t) and a dc-portion g ₀:C (t)= g ₀ +g ₁ ·B (t)+ g ₂ ·B *(t)   (12)

In a good homodyne receiver the modulus of the complex coefficient g ₂is much smaller than the modulus of the complex coefficient g ₁.

Aim of the characterization is the determination of the coefficients g₀, g ₁ and g ₂. However the coefficients b _(m), with m=2n+1, n=0, 1, 2,. . . are not known.

The coefficients are determined by means of known methods of leastsquares, wherein the mean error between the measured receiving signalY(t) and the received signal C(t) which is derived from the transmissionsignal A(t) is to be calculated:

$\begin{matrix}{\overset{\_}{{{e(t)}}^{2}} = {\overset{\_}{{{{\underset{\_}{Y}(t)} - {\underset{\_}{C}(t)}}}^{2}}\overset{\overset{i}{︷}}{=}{\min.}}} & (13)\end{matrix}$

The receiving signal C(t) can be described by the following formula:

$\begin{matrix}{{\underset{\_}{C}(t)} = {{\underset{\_}{g}}_{0} + {{\underset{\_}{g}}_{1}{\sum\limits_{n}{{\underset{\_}{b}}_{{2n} + 1}{{\underset{\_}{A}(t)}}^{2n}{\underset{\_}{A}(t)}\mspace{14mu}{\exp( {j\;\omega_{D}t} )}}}} + {{\underset{\_}{g}}_{2}{\sum\limits_{n}{{\underset{\_}{b}}_{{2n} + 1}^{*}{{\underset{\_}{A}(t)}}^{2n}{{\underset{\_}{A}}^{*}(t)}\mspace{14mu}{\exp( {{- j}\;\omega_{D}t} )}}}}}} & (14)\end{matrix}$

On the basis of this formula and with the following assumptions |g ₀|,|g ₂|«|g ₁| and C(t)□C′(t) and by setting g ₁=1 the coefficients b _(n)are determined according to a first approach:

$\begin{matrix}{{\underset{\_}{C^{\prime}}(t)} = {{\sum\limits_{n}{{\underset{\_}{b}}_{{2n} + 1} \cdot {{\underset{\_}{A}(t)}}^{2n} \cdot {\underset{\_}{A}(t)} \cdot {\exp( {j\;\omega_{D}t} )}}} = {\sum\limits_{n}{{\underset{\_}{b}}_{{2n} + 1} \cdot {\underset{\_}{F_{n}}(t)}}}}} & (15)\end{matrix}$

F_(n)(t) is a substituting function for error determination. Knownmethods for solving the linear problem of the least square can be usedfor determining the coefficients b _(m):

$\begin{matrix}{\overset{\_}{{{e^{\prime}(t)}}^{2}} = {\overset{\_}{{{{\underset{\_}{Y}(t)} - {{\underset{\_}{C}}^{\prime}(t)}}}^{2}}\overset{\overset{i}{︷}}{=}{\min.}}} & (16)\end{matrix}$

The signals are time variant so that it is appropriate to average thesignals for a certain period. The averaging corresponds to an averagingin time or a correlation of the signals for e.g. about 10 ms. Thereceived signal Y(t) comprises besides the reference signal u_(R)(t)also the desired signal u_(S)(t). The cross-correlation of the desiredsignal and the reference signal or the cross-correlation of the desiredsignal and the function F _(n)(t) is negligible small.

In a second step C(t) can be calculated from C′(t) with the followingformula:C (t)= g ₀ +C′ (t)+ g ₂ ·C′ *(t)   (17)

The coefficients g ₀ and g ₂ are calculated with known methods forsolving linear problems of the least square according to the followingformula:

$\begin{matrix}{\overset{\_}{{{e(t)}}^{2}} = {\overset{\_}{{{{\underset{\_}{Y}(t)} - {\underset{\_}{g}}_{0} - {\underset{\_}{C^{\prime}}(t)} - {{\underset{\_}{g}}_{2} \cdot {{\underset{\_}{C^{\prime}}}^{*}(t)}}}}^{2}}\overset{\overset{i}{︷}}{=}{\min.}}} & (18)\end{matrix}$

Alternatively the coefficients g ₀, g ₁, g ₂ and b _(m) are determinedaccording to a second approach with known methods according to solvingthe non-linear problems of least squares and that the following equationis directly minimized:

$\begin{matrix}{\overset{\_}{{{e(t)}}^{2}} = {\overset{\_}{{{{\underset{\_}{Y}(t)} - {\underset{\_}{C}(t)}}}^{2}}\overset{\overset{i}{︷}}{=}{\min.}}} & (19)\end{matrix}$

As the minimization has not to be carried out in real time all knownmethod for solving the non-linear problems of least squares can be used.

An IQ-balance compensation of the received signal Y(t) can be carriedout with the known coefficients g ₀, g ₁, and g ₂ for receiving acorrected receiving signal X(t) according to the following formula:

$\begin{matrix}{{\underset{\_}{X}(t)} = \frac{{{\underset{\_}{g}}_{1}^{*} \cdot {\underset{\_}{Y}(t)}} - {{\underset{\_}{g}}_{2} \cdot {{\underset{\_}{Y}}^{*}(t)}} - {{\underset{\_}{g}}_{1}^{*} \cdot {\underset{\_}{g}}_{0}} + {{\underset{\_}{g}}_{2} \cdot {\underset{\_}{g}}_{0}^{*}}}{{{\underset{\_}{g}}_{1}}^{2} - {{\underset{\_}{g}}_{2}}^{2}}} & (20)\end{matrix}$

The above representation in the time domain can be transformed into adiscretely-timed signal by means of a z-transformation if a scanningfrequency f_(A) is sufficiently large. This is the case if f_(A)>2f_(D).

A potential frequency dependency of the amplitude and phase transmissioncan be generally described by means of a convolution with compleximpulse responses in the time domain instead of a multiplication ofcomplex coefficients. The transfer function with the coefficients b_(m, k) is used instead of b _(m) in a time-discrete formulation bymeans of z-transformation. The calculated received signal C can berepresented in a linear combination in the same way as in theabove-described example, so that the same solution principles can beused. Particularly, the first above described approach is solvable as alinear problem.

This embodiment of the transceiver module 12 comprising the homodynereceiver 65 and the IQ-compensation module 67 allows to create acompensation of the IQ-imbalance during operation, wherein an attenuatedportion of the transmitting signal is coupled from the transmission path30 to the receiving path 45 and after amplification by the homodynereceiver 65 used for determining of IQ-imbalancing coefficients g ₀, g₁, and g ₂. The received and amplified signal Y(t) is compensated withrespect to IQ-imbalance by means of these coefficients. Thus, it ispossible to carry out an IQ-imbalance compensation during the operationof the transceiver module 12.

The above described radio equipment 3 is completely based on electroniccircuits, wherein some parts are digital circuits and other parts areanalog circuits.

In the following, embodiments of the radio equipment 3 are explainedwhich comprise optoelectronic circuits. Such optoelectronic circuits arealso called photonic circuits or microwave photonic circuits.

In “Optical Beam Forming Network created by Silicon Nitride microwavephotonic circuits”, Chris G. H. Roeloffzen et al., Optics Express 2,Vol. 21, No. 19, Sep. 23, 2013 describes several microwave photonicprocessing functionalities based on combinations of Mach-Zehnder andring resonator filters using the high index contrast silicon nitridewaveguide technology. All functionalities are built using the same basicbuilding blocks, namely straight waveguides, phase tuning elements, anddirectional couplers. This document is also incorporated by reference.

David Marpaung et al., “Integrated microwave photonics” in Laser &Photonics Reviews (in the following: LPR), Nov. 20, 2012 provides anoverview about microwave photonics (MWP) and discusses fundamentals ofmicrowave photonics and certain applications.

Charles Middleton et al., “High Performance Microwave Photonic Linksusing Double Sideband Suppressed Carrier Modulation and BalancedCoherent Heterodyne Detection”, to be presented at MILCOM 2009 in Boston(in the following: MILCOM), presents a microwave photonic linkarchitecture that enables high gain and dynamic range, low noise figure,and multi-octave bandwidth operation.

John Michael Wyrwas, “Linear, Low Noise Microwave Photonic Systems usingPhase and Frequency Modulation”, dissertation (University of California,Berkeley), May 11, 2012, Technical Report No. UCB/EECS-2012-89 (in thefollowing: EECS)(www.eecs.berkeley.edu/Pubs/TechRpts/2012/EECS-2012-89.html) discussesmicrowave photonics applications and the elements used therewith.

All these documents are incorporated by reference.

The above described examples of active antenna systems can also bedesigned in an optical domain i.e. with (microwave) photonic components.Before discussing examples based on an optical domain, electro opticalor optical (photonic) components are explained which can be used in aphotonic active antenna systems.

Up-/Down-Conversion

A Mach-Zehnder modulator can be used for carrying out anup-/down-conversion. Alternatively, it is also possible to use anoptical non-linear element for transmitting a light of a first laser.The first laser light is modulated by the light of a second laser. InEECS, page 5, FIGS. 1.4 and 1.5, such elements are shown for modulatinglight which can be used as an up- or down-conversion element. Furthersuitable modulators are shown in FIG. 10 of LPR and FIG. 1 of MILCOM.

Filter Elements, Particularly Suitable for Analog Predistortion

A digital FIR-filter which is suitable for analog predistortion can berealized by analog components, such as direction couplers and delays,wherein Mach-Zehnder modulators and Mach-Zehnder interferometers areused. Such an FIR-lattice filter architecture is e.g. shown in FIG. 4.1,FIG. 4.2 and FIG. 4.3 of EECS. Further suitable filter structures areshown in FIG. 6, FIG. 9, FIG. 14 and FIG. 15 of LPR. An analog bandpassfilter is shown in FIG. 17 and FIG. 19 of LPR. These filter elements aretunable.

Photonic Beam Former

A photonic beam former is known from US 2013/0194134 A1. This documentis incorporated by reference.

All these known elements can be used as up-converters, down-converters,electro-to-optical modulation units, optical-to-electrical modulationunits, tunable filter elements and optical beamformer chips.

In the following, examples of the radio equipment 3 are explainedcomprising such kind of photonic elements.

FIG. 15 shows a first embodiment of the electrooptical radio equipment3. The radio equipment 3 is connected by means of an antennainterconnect 6 to the radio equipment control unit (not shown), which isidentical to one of the above described embodiments. Similar to theabove embodiments, a transmission path 30 and a receiving path 45 extendbetween the antenna interconnect 6 and a duplex filter 15. In thepresent embodiment, the receiving path 45 is based on electroniccircuits only and embodied according to any one of the above describedembodiments.

The transmission path 30 comprises a converter 70 to prepare theincoming data for optical processing. In the particular embodiment, thedata transmitted via the antenna connect 6 are 8/10B-coded. Theconverter 70 converts the 8/10B code into 8 bit signals. This converterbasically converts the digital base band signals so that the signals canbe modulated onto a laser signal. Thus, other types of conversion can besuitable. These signals are further converted by a digital-to-analogconverter 71 (DAC) into analog signals.

The digital-to-analog converter 71 is connected to an optical beamformerchip 72 (OBF). The optical beam former chip 72 comprises anelectric-to-optical-modulator for converting the electrical analogsignal into an optical signal. The optical beam former chip 72 is e.g.embodied as a photonic beamformer chip, which is disclosed in US2013/0194134 A1. The optical beamformer chip 72 is connected to a laser73 which is in the particular embodiment a high-power laser.

The optical beamformer chip 72 applies delays and optionally amodificated amplitude to the antenna signals, so that the antennasignals can be supplied to each antenna 2 with a predetermined oradaptive phase shift. Tunable ring resonators can be used for generatingthe phase shifts. Such ring resonators are usually coupled to a heaterfor adjusting the temperature of the ring resonator and therebyadjusting the electrical length of the ring resonator. In dependence ofthe length of the ring resonator, the resonance frequency of the ringwill be changed. As such, a different frequency of light will couple tothe ring resonator. Tunable power coupler are provided to the ringresonators to alter the amount of power (the “amount” of the opticalsignal) that couples to the ring. Thus, a single ring resonator providesa tunable delay, but it is bandwidth-limited. There is a trade offbetween the maximum delay achievable and the delay bandwidth. This isaddressed by using more than one ring resonator, wherein plurality ofring resonators can be arranged in a cascade.

The optical beamformer chip 72 is connected to an optical up-converter74 which is a mixer or another photonic upconversion element.

The upconverter 74 is followed by an AFILT 75 which is a small signalanalog filter.

The AFILT 75 is followed by an APD 76 which is a photonic analogpredistortion element. By means of the APD 76, the transmission path 30is linearized. In case of static distortions, the APD can be embodied asan equalizer. The APD 76 can also be adaptive, as a feed forwardstructure or by means of a feedback loop that is provided from theoutput of the base band or from the output of the transmission path 30,wherein the feedback can be taken from the output of the optoelectronicsection as it is shown in the following examples.

The APD 76 (Analog Predistortion Element) processes the HF-antennasignals by a combination of numerical operations, such asmultiplications, additions, etc. From Optical Beam Forming Networkcreated by silicon nitride microwave photonic circuits, Chris G. A.Roeloffzen et al., table 1 basic building blocks and their transferfunctions are known, with which someone skilled in the art can composethe numerical operations for carrying out the analog predistortion inthe APD 76. The numerical operations are basically the same as they areknown from electrical predistortion circuits, however, they have to beadapted to the particular distortions in the optical or photonic sectionof the transmission path 30. Due to the large frequency gap between theHF-antenna signals and the carrier signal, such numerical operations canbe carried out with a high quality factor.

The APD 76 is connected to a filter 77 which is in the particularembodiment a high performance filter. This is an integrated filter forbandpass filtering. As the carrier signal is light, there is a hugefrequency gap between the carrier signal and the modulated user signalor antenna signal, respectively. Due to the huge frequency gap, thefiltering both in the AFILT 75 as well and the filter 77 is carried outwith a high quality factor.

The filter elements 75 and 77 are optical filter elements. Such filterelements can be embodied as interference filters, particularly dichroitfilters or thin film filters. Such filter elements can also be adaptive.Such an adaptive filter element is e.g. a Fabry-Pérot interferometer.The quality factor of such interference filters is much higher than theone of electrical filters. Furthermore, the frequency gap between theantenna signal and the carrier signal (light) is very large incomparison to the above described embodiments based on electricalcircuits. Therefore, the selectivity of the optical filters is muchbetter than the one of the electrical filters.

The optical signals output from the filter 77 are demodulated intoelectrical signals by means of a HP-demodulation unit 78. Thedemodulation unit 78 is in the particular embodiment a high performancemodule for converting optical signals into electrical RF-signals.

Preferably, the laser 73 provides light with sufficient power that nofurther power amplification is needed in the transmission path 30. Thus,the antenna signals can be directly applied via the duplex filter 15 tothe antenna 2.

Optionally, a circulator 79 is provided between the duplex filter 15 andthe antenna 2, which prevents that reflected signals are transmittedback into the transmission path 30.

Thus, the optical processing of the antenna signals improves the qualitysignificantly with respect to antenna signals achieved by pureelectrical processing. This is particularly the case if no electricalamplification of the beam-formed signals is needed. An electricalamplification can be omitted if the laser light has a sufficientintensity. Therefore, the laser is a high-power laser source 73.

FIG. 16 shows an additional embodiment of the radio equipment 3 havingan optical transmission path 30. The embodiment according to FIG. 16corresponds substantially to the one of FIG. 15, however, the signalsfor the individual antennas or for individual groups of antennas 2 of anantenna array are separately processed after the beamforming in theoptical beamforming chip 72. Thus, the transmission path 30 comprisesseveral parallel optical process branches 80, each comprising anupconverter 74, an AFILT 75, an APD 76, a filter 77, and a demodulator78. The optical process branches 80 are each connected to a poweramplifier 36. The power amplifier 36 is connected to a duplex filter 15which conducts the antenna signals to an antenna 2.

Due to the amplification by the power amplifiers 36, a laser source 73can be used having less power than the one according to the embodimentof FIG. 15.

The APDs 76 can be embodied identically if it is assumed that thedistortion in each optical process branch 80 is substantially identical.If an adaptive APD 76 is used, then a feedback loop is provided for eachoptical process branch 80. Preferably, the feedback loop includes alsothe corresponding power amplifier 36.

According to a further embodiment (FIG. 17), the optical beamformingchip 72 is arranged behind the upconverter 74 and the APD 76 withrespect to the signal-running direction. The optical beamforming chip 72is connected to several optical process branches 81, each comprising afilter element 77 and a demodulation unit 78. Each optical processbranch 81 is connected to a power amplifier 36.

Also the receiving path 40 of the radio equipment 3 can be embodied withoptical processing modules (FIG. 18). The receiving path 40 comprisesseveral optical input branches 82, each connected to a duplex filter 15of an antenna 2 or of a group of antennas 2.

Each optical input branch 82 comprises an electrical-to-opticalmodulation unit 83, an APD 76 and a filter element 77. Each opticalinput branch 82 is connected with an optical beamforming chip 72. Alaser 73 is provided for inputting a light signal into the opticalbeamforming chip 72.

The output of the optical beamforming chip 72 is connected to adownconverter 84, a filter element 77 and a demodulation unit 78. Thedownconverter can also be placed behind the electrical-to-opticalmodulation unit and before the optical beamforming chip.

The beamforming chip 72 is additionally connected to an input line forinputting the receiving (Rx) beamforming information on the radioequipment control unit 5. By this information, it is possible to combinethe antenna signals of the several antennas, wherein even-coded and/orscrambled signals can be decoded and composed.

A further embodiment, which is shown in FIG. 19, correspondssubstantially to the embodiment of FIG. 18, wherein the APD is locatedbetween the down-converter 84 and the filter element 77 and not in theoptical inbut branches 82.

FIG. 20 shows a further embodiment of a radio equipment 3 comprising anoptical domain receiving path 40 and further comprising means forcalibrating the received antenna signals. This embodiment comprisessubstantially the electrical domain embodiments as shown in FIGS. 9a and9b . the optical domain section in the receiving path 40 issubstantially the same as the one of the embodiment according to FIG.19. Additionally, an external coupling element 53 and an internalcoupling element 56, wherein the internal coupling element 56 is locatedin the receiving path 45 between the duplex filter 15 and theelectrical-to-optical modulation unit 83 and the external couplingelement 53 is located between the duplex filter 15 and the antenna 2.The internal coupling element 56 and the external coupling element 53are connected by a switch 57 to a reference path 46. The reference path46 comprises a digital-to-analog converter 55 for converting a digitalreference signal to an analog reference signal which can be applied toone of the coupling elements 53, 56. The reference signal can begenerated by a digital signal processor and the comparison informationcan be determined by comparing the original reference signal with thereceived signal in a similar way as it is done in the embodiments ofFIG. 9a, 9b . the corresponding means are schematically depicted in FIG.20 by a block for a microcontroller 85. The determined comparisoninformation can be supplied to the APD 76 for calibrating the receivedsignals.

Controlling parameters deducted from the comparison information whichare to be supplied to the APD 76 are to be converted from the electricaldomain to the optical domain by means of an electrical-to-opticalmodulation unit (not shown).

Alternatively, it is possible to calibrate the received signals in thebase band in the radio equipment control unit 5.

Alternatively, it is also possible to couple a portion of thetransmitting antenna signal to the receiving path 45 and using it asreference signal. However, if an optical domain transmitting path 30 isprovided due to the exact bandwidth limitation in an FDD system(frequency domain duplex system), there is hardly any overlap, even inthe harmonics, between the frequencies of the transmitting antennasignals and the frequencies of the receiving antenna signals, so that itits not always not possible to directly use a portion of thetransmitting signals as reference signal. In such a case, additionalspread-spectrum-signals can be used in the transmitting antenna signalsas reference signals, which do as such contain no information. It isalso possible to use gaps in the spectrum between the frequencies or inthe channels outside the regular transmitting frequencies.

If the radio equipment 3 is a TDD system (time domain duplex system),wherein the same frequencies are used for transmitting and receiving,then it is possible to directly use a portion of the transmittingantenna signal as reference signal in the receiving path 45.

FIG. 23 shows an example of a feedback APD 76 which comprises a mainbranch 86 and correction branches 87, which are parallel to the mainbranch 86. The main branch 86 comprises a delay element 88. Eachcorrection branch 87 comprises a polynomial correction element 89.Further delay elements 90 are located in the correction branches 87before or after the polynomial correction element 89. All correctionbranches 87 are combined with an adder 91 for adding the signals of theindividual correction branches 87 to a common correction signal.

The delay elements 90 and the polynomial correction elements 89 areconnected to the microcontroller 85 for receiving correction parameters.Thus, the polynomial correction elements 89 are individually adjustableaccording to the determined comparison information. The correctionbranches 87 are coupled to the main branch 86 before and after the delayelement 88 by means of coupling elements 92, 93. A portion of the signalwhich is to be corrected is branched off in the correction branches 87,where this portion of the signal is processed by several polynomialcorrection elements. The summarized correction signals are then coupledonto the main branch 86 for providing a predistortion to the signalwhich is to be corrected in the main branch 86. The delay elements 88,90 secure that all signals of the main branch 86 and of all correctionbranches 87 are synchronized.

FIG. 22 shows an example of a feed forward APD 76 comprising a mainbranch 94 and a correction branch 95, wherein the correction branch iscoupled to the main branch by coupling elements 96, 97. The main branch94 comprises a delay element 98 and the correction branch 95 comprisesat least one correction element 99, 100, 101. In the particularembodiments, the correction elements 99-101 are a variable phase shiftelement 99, a variable attenuator 100, and a parameterized non-linearityelement 101. These correction elements 99-101 are adjustable, whereinduring use they are adjustable and fixed parameters. It is also possibleto provide several sets of parameters in a look-up table, whereinindependence of the actual status of use the corresponding set ofparameters is loaded to the correction elements 99-101, so that the APD76 is adapted to a predefined operation status. Such an operation statuscan be defined by signal levels, temperature, etc.

FIG. 21 shows a further embodiment of a radio equipment 3 comprising anoptical domain transmission path 30 and further comprising means forcalibrating the transmission antenna signals. This embodiment comprisessubstantially the electrical domain embodiments as shown in FIGS. 8a and8b . the optical domain section in the transmission path 30 issubstantially the same as the one of the embodiment according to FIG.17. A feedback loop extending from the couplers 53, 56 is provided witha reference signal from the output of the base band. The referencesignal is generated by a microcontroller 85.

The photonic APD 76 used in the above described embodiments of a radioequipment 3 is an analog predistortion element 76. A predistortionelement can be also embodied as a FIR filter or IIR filter which isusually a digital element. Therefore, the photonic predistortion elementcan also be a digital predistortion element (DPD).

The above described examples according to electrical as well as photonicdomain are embodied for FDD (frequency division duplex), wherein thetransmitting and the receiving signals are transmitted in differentfrequency bands. These embodiments can also be embodied for TDD (timedivision multiplex), wherein the same frequencies are used fortransmitting and receiving the antenna signals. In such a case theduplex filter is to be replaced by a switch for connecting alternativelythe transmitting paths and the receiving paths with the correspondingantennas.

The above embodiments comprises an antenna interconnect 6 which isembodied for transmitting data according to the CPRI standard. Theantenna interconnect can also be embodied for transmitting dataaccording to other standards or other protocols, such as ORI, OBSAI,etc. The antenna interconnect can also be embodied for transmittinganalog signals.

LIST OF REFERENCE SIGNS

-   1 active antenna system-   2 antenna-   3 radio equipment-   4 antenna tower-   5 radio equipment control unit-   6 antenna interconnect-   7 Interface-   8 core controller-   9 channel card-   10 antenna cable-   11 hub-   12 transceiver module-   13 transceiver filter unit-   14 control RF-base band unit-   15 duplex filter amplifier-   16 Tx-transmission chip-   17 Rx-receiving chip-   18 RF-base band chip-   19 digital filter-   20 calibration module-   21 controller-   22 transport interface-   23 antenna module-   24 antenna array-   25 switch matrix-   26 terminal-   27 intermediate cable-   28 main lobe-   29 side lobe-   30 transmission path-   31 reference path-   32 digital predistorsion unit-   33 intermediate frequency-up mixer-   34 digital-to-analog converter-   35 channel frequency-up mixer-   36 power amplifier-   37 coupling element-   38 intermediate frequency-down mixer-   39 analog-to-digital converter-   40 base band down mixer-   41 digital signal processor-   42 calibration unit-   43 internal coupling element-   44 switch-   45 receiving path-   46 reference path-   47 low noise amplifier-   48 channel band filter-   49 intermediate frequency-down mixer-   50 intermediate frequency band filter-   51 base band down mixer-   52 analog-to-digital converter-   53 coupling element-   54 channel frequency-up mixer-   55 digital-to-analog converter-   56 internal coupling element-   57 switch-   58 amplifier-   59 summarizing node-   60 summarizing node-   61 current source-   62 switch-   63 communication partner-   64 cell-   65 homodyne receiver-   66 attenuation element-   67 IQ-compensation module-   68 IQI-coefficient determination module-   69 RNC-   70 converter-   71 digital-to-analog converter-   72 optical beamformer chip-   73 laser-   74 upconverter-   75 AFILT-   76 APD-   77 filter element-   78 demodulation unit-   79 circulator-   80 optical process branch-   81 optical process branch-   82 optical input branch-   83 electrical-to-optical modulation unit-   84 downconverter-   85 microcontroller-   86 main branch-   87 correction branch-   88 delay element-   89 polynomial correction element-   90 delay element-   91 adder-   92 coupling element-   93 coupling element-   94 main branch-   95 correction branch-   96 coupling element-   97 coupling element-   98 delay element-   99 radiator

What is claimed is:
 1. Method for establishing a secure data connectionusing an active antenna system having a plurality of antennas fortransmitting signals via a plurality of radio-channels, wherein theradio-channels can be beamformed, comprising: scanning for acommunication partner, estimating the direction of arrival (DOA) of asignal transmitted from a recognized communication partner, sub-dividinga signal s(t) containing certain data which are to be transmitted to therecognized communication partner in several partial signals d(t),wherein the partial signals d(t) are distributed onto a certain number(N) of radio-channels and transmitted via a beamformed signal based onthe estimated DOA.
 2. The method according to claim 1, furthercomprising the step of transmitting an encryption key with the signals(t).
 3. The method according to claim 1, further comprising the step ofcalculating predistortion factors(wl, . . . , wN) are calculated forcompensating distortion in the individual radio channels.
 4. The methodaccording to claim 3, wherein weighting factors for inverselyreproducing the radio channels (wl, . . . , wN ⇄h-11, . . . , h-1N) arecalculated as the predistortion factors, and these weighting factors arecombined with beam-forming parameters to a beam-forming matrix. 5.Method according to claim 1 using an active antenna system having aplurality of antennas for transmitting signals via a plurality ofradio-channels, the method further comprising: measuring certainphysical parameters of the radio channels and using the physicalparameters for encrypting data to be exchanged between the activeantenna system and the communication partner.
 6. The method according toclaim 5, wherein the active antenna system further comprises: a radioequipment comprising a plurality of transceiver modules, eachtransceiver module being connected to at least one antenna, a radioequipment control unit, wherein each transceiver module comprises aswitch matrix being connected to the radio equipment control unit via anantenna interconnect for transmitting data between the switch matrix andthe radio equipment control unit.
 7. The method according to claim 6,wherein the antenna interconnect comprises antenna cables for connectingthe transceiver modules with the radio equipment control unit or whereinthe antenna interconnect is a radio connect.
 8. The method according toclaim 7, wherein the antenna cables are embodied as glass-fiber cables,wherein the data signals are transmitted in one color or in multiplecolors (DWDM).
 9. The method according to claim 7, wherein the antennacables are embodied as conducting wires, glass-fiber cables, wherein thedata signals are transmitted in one color or in multiple colors (DWDM).10. The method according to claim 6, wherein the antenna interconnect isembodied for a serial data transmission, as a serial data bus.
 11. Themethod according to claim 6, wherein the radio equipment comprises anup-converter for converting a base band signal to a high frequency RFantenna signal for transmitting the antenna signal via the correspondingantenna, and a down-converter for converting a received high frequencyRF antenna signal to a base band signal.
 12. The method according toclaim 6, wherein the radio equipment comprises a digital-to-analogconverter for converting a. digital signal received from the radioequipment control unit into an analog signal, and an analog-to-digitalconverter for converting an analog signal received by means of theantenna into a digital signal, wherein digital signals are transmittedvia the antenna interconnect.
 13. The method according to claim 12,wherein the digital signals of or for several antennas are jointlycompressed.
 14. The method according to claim 13, wherein the jointlycompressed digital signals are beamforming signals.
 15. The methodaccording to claim 5, wherein the active antenna. system furthercomprises: a plurality of antennas, a radio equipment connected to theantennas and being located adjacent to the antennas, wherein the radioequipment comprises for each antenna a separate transceiver module. 16.The method according to claim 15, wherein a radio equipment control unitis connected to the radio equipment by an antenna interconnect fortransmitting data between the radio equipment and the radio equipmentcontrol unit.
 17. The method according to claim 16, wherein the antennainterconnect comprises a glass-fiber cable, wherein the data signals aretransmitted in one color or in multiple colors (DWDM).
 18. The methodaccording to claim 16, wherein the antenna interconnect is embodied fora serial data transmission, as a serial data bus.
 19. The methodaccording to claim 15, wherein the active antenna system furthercomprises an up-converter for converting a base band signal to a highfrequency RF antenna signal for transmitting the antenna. signal via thecorresponding antenna, and a down-converter for converting a receivedhigh frequency RF antenna signal to a base band signal.
 20. The methodaccording to claim 16, wherein the radio equipment comprises adigital-to-analog converter for converting a digital signal receivedfrom the radio equipment control unit into an analog signal, and theradio equipment comprises an analog-to-digital converter for convertingan analog signal received by the antenna into a digital signal.
 21. Themethod according to claim 20, wherein digital signals are transmittedvia the antenna interconnect, wherein the digital signals of or forseveral antennas are jointly compressed.
 22. The method according toclaim 1, wherein the jointly compressed digital signals are beamformingsignals.
 23. The method according to claim 5, wherein a radio equipmentcontrol unit of the active antenna system comprises a hub.
 24. Themethod according to claim 23, wherein a radio equipment of the activeantenna system is free of any hub.
 25. The method according to claim 23,wherein several antenna modules of the radio equipment each comprise aswitch matrix.
 26. The method according to claim 25, wherein the switchmatrix can be connected to the radio equipment control unit by anantenna interconnect or to a switch matrix of another antenna module byan intermediary cable.
 27. The method according to claim 1, wherein anactive antenna system is used, the active antenna system comprising: aradio equipment having a plurality of transceiver modules, eachtransceiver module being connected to at least one antenna, a radioequipment control unit having a hub being connected to the transceivermodules via an antenna interconnect, wherein the hub is embodied forreceiving base band signals via the antenna interconnect from thetransceiver modules and to extract channel signals from the receivedbase band signals.
 28. The method according to claim 1, wherein anactive antenna system is used, the active antenna system comprising: aplurality of antennas, a radio equipment connected to the antennas andbeing located adjacent to the antennas, wherein the radio equipmentcomprises for each antenna a separate transceiver module and antennamodules are each comprising one of said antennas and one of saidtransceiver modules, further comprising modules for determining delayscaused by the runtime of antenna signals from a data source to therespective antenna module and for calculating delay coefficients forcompensating the delays caused by the respective runtime.
 29. The methodaccording to claim 1, wherein an active antenna system is used, theactive antenna system comprising: a plurality of antennas, a radioequipment connected to the antennas and being located adjacent to theantennas, wherein the radio equipment comprises an optical beam formerchip and a photonic predistortion element (API) or DPD).
 30. The methodaccording to claim 1, wherein the recognized communication partnerreceives the signal s(t) by superimposing the partial signals d(t). 31.The method according to claim 30, wherein a complete superposition toproduce the signal s(t) is only possible if the recognized communicationpartner is in a location corresponding to the estimated direction ofarrival for the recognized communication partner.
 32. The methodaccording to claim 31, wherein a signal resulting from superposition ofthe partial signals d(t) received from a location other than thelocation corresponding to the estimated direction of arrival for therecognized communication partner is substantially different from thesignal s(t).